Sequence time domain reflectometry using complementary golay codes

ABSTRACT

A method and system for performing sequence time domain reflectometry over a communication channel to determine the location of line anomalies in the communication channel is disclosed. In one embodiment, the system generates a sequence signal and transmits the sequence signal over an optical channel. The system receives one or more reflection signals over the optical channel and performs reflection signal processing on the reflection signal. In one embodiment, the optical reflection is transformed to an electrical signal and correlated with the original sequence signal to generate a correlated signal. The time between the start of the reflection signal and a subsequent point of correlation and the rate of propagation reveals a line anomaly location. A circulator, beam splitter, or any other similar device may direct the reflection signal to the apparatus configured to perform reflection signal processing.

This application is a continuation-in-part of application Ser. No.10/095,825, entitled Optical Time Domain Reflectomentry, filed Mar. 11,2002, which is a continuation-in-part of application Ser. No.09/810,932, entitled Method and Apparatus for Transmission Line Analysisfiled Mar. 16, 2001.

FIELD OF THE INVENTION

This invention relates generally to communications and in particular toa system and method for analyzing a transmission line.

RELATED ART

Historically, new communication technologies are continually beingintroduced to improve the ease and rate at which data can be exchangedbetween remote locations. One factor that must be considered whencommunicating electronic data is the medium over which the data willtravel. This is often referred to as determining the channel quality,line characteristics, line transfer function, insertion loss, or channelimpulse response. Numerous different types of conductors are utilized toconduct communication signals. One example medium that is commonlyinstalled throughout the world is twisted pair conductors as aretraditionally used to provide telephone service between a central officetelephone facility and a residence or business.

The medium must be considered because the medium and its condition canaffect the rate at which communication may occur. For example, digitalsubscriber line (DSL) technology utilizes twisted pair conductors. Therate at which systems using the DSL standards may operate is determinedin part by the electrical characteristics of the twisted pair between atransmitting device and a receiving device. The factors that control therate of communication may include the distance between the receiver andtransmitter, presence of bridge taps or load coils, the quality of thetwisted pair, the quality of connections to the twisted pair, and theamount of noise that the twisted pair picks up, such as crosstalk noise.As data communication speeds increase, the quality of the line and thepresence of line anomalies become of greater importance.

It may be desirable to determine characteristics of the line prior tocommunicating data so that a data transmission rate may be determined orso that it may be determined if the line is able to supportcommunications under a particular standard. For example, if certain lineanomalies exist between a first communication unit and a secondcommunication unit, it is desirable to learn of these anomalies andtheir effect on communication through the line. Moreover, it isdesirable to determine the location of the anomalies so that repair orremoval of the anomaly may occur. In the particular case of bridge tapsand load coils, service technicians are dispatched to locate and removethe bridge tap or load coil. The dispatch of service technicians isexpensive and hence, the less time the service technician must spendlocating the line anomaly, the lower the cost of the dispatch.Therefore, the more accurately the anomaly location is identified, theless costly the service dispatch because the technician may more rapidlyfind and fix the anomaly.

One prior art method of line analysis, such as for evaluating theeffects of or identifying the location of line anomalies comprisestransmission of a high power pulse on the line. Impedance irregularitiesin the line cause a reflection or echo when encountered by the pulse.Time information is used to determine the location of the anomaly.

This method of line analysis suffers from numerous disadvantages. Onedisadvantage arises as a result of the necessary, but undesirable, useof a high power pulse. Transmission of a high power pulse on a linedisrupts communication and operation of the other pairs in the binder bycreating crosstalk between pairs. Another disadvantage of this prior artmethod arises because of the available echo processing methods. Thein-use pairs in the binder with the line being tested create crosstalkin the line being tested. This limits the detectability of weak returnechoes which translate into a limitation on the ability of prior artpulse system to accurately analyze the distant end of a long line. Yetanother drawback associated with the prior art method of high powerpulse reflection analysis is the limited platforms available to generatea high power pulse. As a result, pulse test equipment must beimplemented as a separate piece of test equipment and may not be anintegrated circuit. This increases the cost of testing by requiring aseparate piece of test equipment and can make its use inconvenient.

In the case of line analysis of an optical fiber or cable, the prior artis limited by the magnitude or power of a pulse that may be sent overthe fiber. As a result, the resolution and strength of the system isundesirably limited. The method and apparatus described and claimedbelow overcomes these drawbacks.

The invention overcomes the disadvantages of the prior art by providinga method for apparatus for sequence time domain reflectometry.

SUMMARY

In one embodiment, the invention comprises a line probe signal andmethod of generating the same for use in determining linecharacteristics. In one embodiment, the invention comprises a method andapparatus for processing a line probe signal to determine channelcharacteristics, such as to determine the location and type of one ormore line anomalies. Line anomalies may comprise open circuit, shortcircuit, bridge taps, load coils, moisture on the line, or any otheraspect that creates an impedance mismatch.

In one embodiment, a method for performing time domain reflectometry ona communication channel comprises generating a sequence signal andtransmitting the sequence signal over a communication channel. In oneembodiment the sequence signal has an autocorrelation function, whichapproximates a Kronecker delta function. The communication channel maycomprise any channel including, but not limited to, fiber optical cable,coaxial cable, power transmission line, network line Ethernet, twistedpair or any channel capable of conducting data. The length of the lineor channel being analyzed may range from fractions of a millimeter tothousands of miles. Next, the system receives one or more reflectionsignals from the communication channel in response to the transmissionof the sequence signal. After receipt of the reflection signal, thesystem correlates the reflection signal with the sequence signal togenerate a correlated signal. Due to the autocorrelation properties ofthe sequence signal, the correlated signal is a linear combination ofthe near-end echo and the echoes from one or more anomalies. Next, thesystem may retrieve a template signal. The template signal correspondsor is representative of the near-end echo in the reflection signal.After retrieving the template signal, the system aligns the templatesignal and the correlated signal to determine a point of alignment. Thepoint of alignment may comprise when the two signal are most similar.Once aligned, the method subtracts the template signal from thecorrelated signal to remove near-end echo from the correlated signal.Other aspects of the reflection signal may be removed other than near-end echo. Next, the system measures a time interval between thepoint of alignment and a subsequent peak in the correlated signal. Thisreveals the amount of time it took for the signal to propagate to a lineanomaly and for the reflection signal to return to the receiver. Whenthe propagation time is determined, the system multiplies the timeinterval by the rate of propagation of the sequence signal through thecommunication channel to obtain a distance to a line anomaly. The rateof propagation for an electrical signal through a channel is generallyknown for different channel mediums. In one embodiment the method of theinvention further includes dispatching a service technician or otherpersonnel to fix the line anomaly.

In various other configurations or embodiments, the template signal maybe measured or created by correlating the reflection from a long cableof the type to be tested and known to be free of a nomalies or thetemplate may be derived from a detailed circuit analysis of thetransceiver and the line interface. In one embodiment, the sequencesignal is transmitted at a power level that does not introduce crosstalkinto other communication channels.

In another variation or embodiment, the method of operation alsoperforms a circular rotation of the sequence signal to create a rotatedsequence signal and transmits the rotated sequence signal over thecommunication channel. A rotated reflection signal is received andcorrelated with the rotated sequence signal that was transmitted tocreate a rotated reflection signal. This correlated rotated signal isaligned with the correlated signal and combined with the correlatedsignal to reduce or remove correlation artifacts on the correlatedsignal.

To realize this method of operation, various different configurations ofhardware and/or software may be utilized. In one embodiment, a systemperforms sequence time domain reflectometry to determine the location ofimpedance mismatches on a channel being configured to communicate datausing a digital subscriber line standard. This embodiment comprises asequence generator configured to generate a maximal length sequencesignal connected to a transmitter that is configured to transmit thesequence signal on a channel. This causes the sequence signal topropagate through the channel, the channel being analyzed to determinethe location of impedance mismatches that may affect data transmission.A receiver is configured to receive one or more reflections that resultfrom the sequence signal encountering impedance mismatches as itpropagates through the channel. A correlator connects to the receiverand correlates the received signal, which is comprised of one or morereflections, with the sequence signal to generate a correlated signal,which is the linear combination of the impulse responses of thetransmission paths to the one or more anomalies. Also included in thisconfiguration is a processor, other hardware or software, configured todetermine the time period between a beginning of the sequence signaltransmission as determined from the peak of the n ear-end echo responseand the peak of the echo response from the one or more anomalies. Theprocessor, other hardware or software, is configured to calculate avalue corresponding to a channel length between the system and animpedance mismatch.

The invention can be implemented from only one end of the channel, suchas when access is possible or convenient to only one end, or wheninvention may be performed at any point along the channel. The inventionmay be used to classify the line or channel into a data transmissionrate group, a cost of service group, or simply whether or not to use theline for high speed data communication. In one embodiment the distortionof the pulse by the transmission medium may be analyzed to discriminatebetween various types of anomalies.

In one or more other embodiments, the system may include various otherfeatures or aspects. In one embodiment, the system is embodied on acommunication device configured to communicate data using a digitalsubscriber line standard. In one embodiment the sequence generatorcomprises a tapped delay line.

In one embodiment, the invention utilizes an echo cancellation method ofoperation to perform time domain reflectometry processing. A method ofoperation based on this alternative embodiment includes processing areflection signal resulting from transmission of a sequence of bits overa channel to determine the location of line anomalies. This occurs byproviding the generated sequence (not correlated with the transmitsequence) to a prediction module, which, in one embodiment, is comprisedof a finite impulse response, adaptive filter. The coefficients of theprediction filter are adapted such that the output of the predictionfilter approximates the received reflection sequence when the transmitsequence is applied to the input of the filter. When this adaptiveprocedure converges, the coefficients of the adaptive filter are anestimate of the linear combination of the near-end echo response and theresponses from the one or more anomalies

This method of operation may further include analyzing the coefficientvalues when the prediction filter output generally resembles thereflection signal to determine the location of impedance mismatches onthe channel. In one particular embodiment the prediction filtercomprises a finite impulse response filter. In one embodiment thesequences of bits may comprises a sequence selected from the group ofsequences consisting of a maximal length sequence, a Barker code, or aKasami sequence. It should be noted that comparing may includesubtracting the prediction filter output from the reflection signal.

In various embodiments the method and apparatus as contemplated by theinvention may be configured to analyze an optical fiber. In such anembodiment the system includes an optical driver configured to transformthe sequence signal into a signal suitable for driving an optical signalgenerator and an optical generator configured to receive the output ofthe optical driver and generate an optical signal. An optical interfaceis provided to route the optical signal from the optical generator to anoptical fiber and output an optical reflection received over the opticalfiber. An optical detector is included and is configured to receive theoptical reflection from the optical interface and convert the opticalreflection to a reflection signal in electrical form.

In one embodiment a near-end echo reduction module configured to removenear-end echo from the reflection signal. In another embodiment theoptical generator comprises a light emitting diode or a laser.

In yet another embodiment an optical sequence time domain reflectometrysystem is provided that comprises a sequence signal source configured toprovide a sequence signal to an optical transmit system. The opticaltransmit system is configured to receive the sequence signal from thesequence signal source, convert the sequence signal to an opticalsignal, and transmit the optical signal through an optical fiber. Such asystem may further include an optical receive system configure toreceive an optical reflection signal and convert the optical reflectionsignal to an electrical reflection signal and a correlator configured toreceive the electrical reflection signal and correlate the electricalreflection signal with the sequence signal.

In one embodiment this system further includes an optical interfacepositioned to interface the optical transmit system and the opticalreceive system with the optical fiber. The system may be configuredwithin a communication device or test equipment. The optical interfacemay comprise a circulator or a beam splitter.

A method may also be provided for determining the location of a lineanomaly in a fiber optic cable. The method may includes the steps ofobtaining a sequence signal, converting the sequence signal into a lightsignal, transmitting the light signal through an optical fiber, andthereafter receiving and directing a reflected light signal to anoptical detector. Next, the method converts the reflected light signalto a reflection sequence in electronic form, correlates the reflectionsequence with the sequence signal to obtain a correlated signal, andanalyzes the correlated signal to determine a point of correlation.Thereafter, the method calculates a duration of propagation of thesequence signal through the optical fiber and calculates a location of aline anomaly based on the duration of propagation and a rate ofpropagation.

In one embodiment the steps of receiving and directing a reflected lightsignal to an optical detector is performed by a beam splitter. In oneembodiment the method may further comprise subtracting a near-end echotemplate signal from the correlated signal to remove an unwanted pointof correlation caused by near-end echo. This method may be performed bytest equipment or communication equipment.

As an alternative, the method may analyze an optical fiber by firsttransmitting a sequence signal through an optical fiber, then receivinga reflection signal from the optical fiber and correlating thereflection signal to create a correlated signal and thereafter,processing the correlated signal to obtain information regarding theoptical fiber. In this method the processing may include subtracting acorrelated near-end echo signal form the correlated signal to remove thepoint of correlation created by the near-end echo so that a point ofcorrelation may be identified. Based on this point of correlation themethod calculates a propagation duration between transmission of thesequence signal and receipt the portion of the reflection that createsthe point of correlation and multiplies the propagation duration with arate of propagation of the sequence signal through the optical fiber.This allows one to determine information concerning a distance to alocation in the optical fiber that created the point of correlation.This method may occur multiple times to obtain numerous referencepoints.

The method and apparatus of the invention may also be realized as acomputer program product comprising a computer useable medium havingcomputer program logic recorded thereon for optical fiber analysis. Onesuch embodiment of such a configuration comprises computer program codelogic configured to generate a sequence signal and an optical generatorconfigured to transmit the sequence signal, in optical form, over anoptical fiber. An optical receiver detects an optical reflection andconverts the optical reflection to a reflection signal in electricalformat. Additional computer program code logic is configured tocorrelate the reflection signal with the sequence signal to create acorrelated signal while other computer program code logic is configuredto analyze the correlated signal to determine a portion of thecorrelated signal having a maximum magnitude. Further computer programcode logic determines a distance to a line anomaly based on a time ofreceipt of the portion of the correlated signal having a maximummagnitude.

Additional computer program product may be configured to analyze thereflection signal to determine the type of line anomaly that is creatingthe reflection signal. This embodiment may include an optical interfaceconfigured to direct a portion of the sequence signal to the opticaldetector and a portion of the sequence signal to the optical fiber. Inone embodiment the computer program code logic configured to determine adistance to a line anomaly comprises computer program code logicconfigured to multiply the time duration for the sequence signal totravel to the line anomaly by one-half the rate of propagation for thesequence signal.

In yet another embodiment, the method and apparatus contemplated by theinvention comprises a method for processing a reflection signal toobtain information about an optical fiber. This method monitors for areflection signal received over an optical fiber. The reflection signalis generated by transmission of an original signal. Upon receipt, themethod correlates the reflection signal with the original signal togenerate a correlated signal and analyzes the correlated signal forpoints of correlation to determine if line anomalies are present in theoptical fiber. In addition, the method may further include analyzing thecorrelated signal to determine the type of line anomaly present in theoptical fiber.

The scope is not limited to only the described combinations but isintended to cover any various combination as might be contemplated afterreading the specification and claims. Hence an embodiment may includeone feature or element or any combination or number of features orelements.

Other systems, methods, features and advantages of the invention will beor will become apparent to one with skill in the art upon examination ofthe following figures and detailed description. It is intended that allsuch additional systems, methods, features and advantages are includedwithin this description, are within the scope of the invention, and areprotected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The components in the figures are not necessarily to scale, emphasisinstead being placed upon illustrating the principals of the invention.Moreover, in the figures, like reference numerals designatecorresponding parts throughout the different views.

FIG. 1 is a block diagram of an example environment of use of theinvention.

FIG. 2A illustrates a block diagram of example embodiment in relation toa communication line and an example line anomaly.

FIG. 2B illustrates a plot of an example reflection signal as may begenerated by and correspond to the exemplary embodiment of FIG. 2.

FIG. 3 illustrates a plot of an example sequence transmission pattern.

FIG. 4 illustrates a more detailed block diagram of an exampleembodiment of one configuration of the invention.

FIG. 5 illustrates a block diagram of an example embodiment of asequence generator configured using a linear feedback shift registertype implementation.

FIG. 6 illustrates an alternative embodiment of a sequence generatorcomprising a tapped delay line configuration.

FIG. 7 illustrates block diagram of an example configuration of acorrelation unit.

FIGS. 8A and 8B illustrate example plots of a sequence signal andassociated noise before and after the correlation operation.

FIG. 9 illustrates a plot of an example sequence signal.

FIG. 10 illustrates an alternative embodiment of the invention.

FIG. 11 illustrates a block diagram of an example configuration of aprediction filter configured to match the channel response.

FIG. 12 illustrates an example method of operation of one embodiment ofthe invention.

FIG. 13 illustrates an example method of sequence generation.

FIG. 14 illustrates an alternative method of sequence generation for usewith a table look-up method.

FIG. 15 illustrates an operational flow diagram of an example method ofcorrelation and processing of reflection signals.

FIG. 16 illustrates an operational flow diagram of an exemplary methodof artifact reduction.

FIG. 17 illustrates an operation flow diagram of an example method ofnear-end echo reduction.

FIG. 18 illustrates an example method of processing the sequence signalto determine the location of line anomalies.

FIG. 19 illustrates an example method of operation for sequence timedomain reflectometry using echo cancellation.

FIG. 20 illustrates a block diagram of an example embodiment of anoptical sequence time domain reflectometry system configured as orcontained in test equipment.

FIG. 21 illustrates a block diagram of an example embodiment of anoptical sequence time domain reflectometry system configured as orcontained in a communication device.

FIG. 22 illustrates a block diagram of an example embodiment of anexample implementation of an optical sequence time domain reflectometrysystem.

FIG. 23 illustrates a block diagram of an alternative example embodimentof an example implementation of an optical sequence time domainreflectometry system.

FIG. 24 illustrates a block diagram of an example implementation of anoptical sequence time domain reflectometry system equipped with a beamsplitter.

FIG. 25 illustrates a block diagram of a functional representation of anoptical interface.

FIG. 26 illustrates an operational flow diagram of an example method ofoptical sequence time domain reflectometry.

FIGS. 27A and 27B illustrate an operational flow diagram of an examplemethod of operation.

DETAILED DESCRIPTION

FIG. 1 illustrates an example environment for use of the invention. Theexample environment shown in FIG. 1 is provided for purposes ofdiscussion and is not in any way intended to limit the scope or breadthof the invention. It is contemplated that the invention may find use ina plurality of other environments, such as any environment where it isdesired to obtain information regarding line characteristics for thepurposes of communication over the line, line repair or lineclassification. The line to be probed may comprise any type of conductoror channel including, but not limited to, a twisted pair conductor,coaxial cable, Ethernet, an optical channel, or a radio frequencywaveguide.

FIG. 1 illustrates customer premise equipment (CPE) 100 in communicationwith a communication interface 102 over a first line 104. The CPE 100comprises any communication device that is generally located remote fromthe communication interface 102 and configured to facilitatecommunication over the first line 104. In one embodiment, the CPE 100comprises a communication modem or communication device located at abusiness or residence. The CPE 100 may comprise, but is not limited to,any device operating under the digital subscriber line (DSL) standard,any voice band modem, cable modem, wireless modem, power line modem, orany other device configured to perform digital or analog communication.It is contemplated that contained in the CPE 100 and the communicationinterface 102 there is a receiver and transmitter configured to send andreceive data over the line 104.

The first line 104 may comprise any communication medium intended tocarry communication signals. In various embodiments the first line 104comprises, but is not limited to, one or more conductors of a twistedpair of conductors, coax cable, power line, optical cable. Although thefirst line 104 is shown as a single line, it should be understood thatthe line 104 may comprise any configuration or number of conductors,optical paths, or other such paths. Other lines, channel, or paths orconductors shown throughout the figures may likewise comprise anyconfiguration or number of conductors, optical paths, or other suchpaths.

In this embodiment, the communication interface 102 comprises anycommunication equipment configured to communicate with the CPE 100 overthe first line 104. With regard to the DSL standard, the communicationinterface 102 may comprise a digital subscriber line access multiplexer(DSLAM). A DSLAM is configured to facilitate communication over thefirst line 104 between the CPE 100 and a central office (CO) switch 106and an Internet Service Provider (ISP) 110. The DSLAM may include modemsor other communication devices.

Communication with the CO switch 106 occurs over a second line 108 whilecommunication with the ISP 110 occurs over a third line 112. Thecommunication interface 102 appropriately routes certain voicecommunication from the CPE 100 to the CO switch 106 while appropriatelyrouting certain data communication from the CPE to the ISP 110. Asshown, the CO switch 106 may connect to the PSTN 116 thereby serving asa switching and routing service for telephone, facsimile, or data calls.The ISP 10 may connect to the Internet 118 to provide access to aplurality of other networked computers.

It is contemplated that the various embodiments of the invention may beused to evaluate the characteristics of the first line 104, the secondline 108, the third line 112, or lines 120 and 122 to thereby determinecharacteristics of the line, such as but not limited to the location ofline anomalies that may effect data transmission. It is desired toobtain the highest data rate supportable by the lines 104, 108, 112,120, 122 so that a maximum amount of data may be transferred in aminimum amount of time with the fewest number of errors. This enablesmore rapid upload, downloads, and greater and more reliable use of thelines 104, 108, 112, 120, 122. It is also contemplated the invention maybe practiced at any location in the communication system. In onepreferred embodiment, the invention is integrated with modems at thecommunication interface 102, the C.O. switch 106, or the communicationinterface 102. The invention may also be used to determine a linecharacteristics for each leg or path for symmetrical communication(identical or similar data transmission rates between devices) orasymmetrical communication (different data transmission rates betweendevices).

FIG. 2A illustrates a block diagram of example configuration in relationto a communication line and an example line anomaly. A sequencegenerator and transmit module 200 connect to a hybrid 204. The hybrid204 connects to channel 208. In one embodiment, the channel 208comprises a twisted pair conductor. In another embodiment, the conductorcomprises fiber optic cable. In yet another embodiment, the channel maycomprise coaxial cable or radio waveguide.

The opposite end of the channel 208 connects to a line termination 216.It is contemplated that the line termination 216 may comprise an opencircuit, short circuit, or a termination impedance matched to the line.Both an open circuit and a short circuit create reflections. Althoughnot the case in every channel, for purposes of understanding a lineanomaly 220 resides between the hybrid 204 and the line termination 216.In the embodiment shown in FIG. 2, a conductor 212 is spliced into thechannel 208 forming a bridged tap. The bridged tap is typicallyterminated in an open circuit. In one embodiment, the conductor 212comprises the same type channel material as the channel 208. In anotherembodiment, the conductor 212 comprises the same general class oftransmission line with slightly different properties, such as wiregauge. Other line anomalies may include an open circuit or a shortcircuit in the channel cable 208. It is contemplated that the lineanomaly may be located at any distance from the point at which the testis applied and, in the case of a bridged tap, the spliced cable may beof any length. Hence, the conductors 208 and 212 may assume any length.

The hybrid 204 also connects to a receiver and reflection module 224that is configured to monitor for and receive reflection signals fromthe hybrid arriving over the channel 208.

The sequence generator and transmit module 200 comprises a configurationof software, hardware, integrated circuit, analog system, or somecombination thereof that is collectively configured to generate asequence signal in accord with the teaching discussed below. It iscontemplated that the sequence generator portion of the module 200generates a sequence signal for transmission over the channel 208. Asdiscussed below in greater detail, the sequence signal has numerousadvantages over the prior art use of a single high power pulse when usedfor time domain reflectometry. Any type of sequence signal may beutilized and is compatible with and covered by the scope of theinvention. In one embodiment, a sequence signal with goodautocorrelation properties is used. In one embodiment it is desired tohave a signal with a generally flat frequency response across thefrequency spectrum that will be used for communication. Severaldifferent example sequence signals are provided below for purposes ofunderstanding. However, the invention is not limited to the specificsequences specified in this document. Further, numerous different typesof transmitters, modulator, filters and other transmit components may beadopted for use and are contemplated as being covered by the claims. Theinvention is not limited to any particular type of transmit system.

In one embodiment, the sequence generator portion of the module 200 isembodied in software and configured to execute in conjunction with aprocessor to generate a physical electrical signal. Any type ofprocessor, hardware, or integrated circuit may execute the softwarecode. The software may be stored in memory or any computer readablemedium.

The hybrid 204 operates as understood in the art. It is designed toallow the signal received from the channel 208 to pass through to thereceiver but to minimize the amount of the transmit signal which isdirectly coupled into the receiver. There is, in general, some residualsignal directly coupled to the receiver and this is termed the near-endecho.

The line anomaly comprises any connection, break, disruption or aspectthat creates an impedance mismatch. When encountered by a signal, suchas a sequence signal, this mismatch creates a reflection that echoesback in the direction of the received signal. In one embodiment, theline anomaly 220 comprises a load coil. In another embodiment, the lineanomaly 220 comprises a bridge tap. Other line anomalies include but arenot limited to, semi open or short circuits, moisture or corrosion onthe linear a change in wire gauge.

The receiver and reflection processor 224 comprises a configuration ofsoftware, hardware, integrated circuit, analog system, or somecombination thereof that is collectively configured to receive areflection sequence signal and process the reflection sequence signal toobtain information regarding anomalies on the channel 208. As discussedbelow in greater detail, the sequence signal has numerous advantagesover the prior art signal of a high power pulse when used for timedomain reflectometry. Any type of reflected sequence signals may beprocessed and is contemplated as being compatible with and covered bythe scope of the invention. Further, numerous different types ofreceivers, demodulator, filters, or other transmit components may beadopted for use. The invention is not limited to any particular type ofreceiving system.

In one embodiment, the receiver and reflection processor 224 is embodiedin software and configured to execute on a processor. Any type ofprocessor, hardware or integrated circuit may be used to execute thesoftware code. The software may be stored in memory or any computerreadable medium.

FIG. 2B illustrates an example plot of a reflection signal as may begenerated by and correspond to the exemplary configuration of FIG. 2A.FIG. 2B is described with reference to FIG. 2A. The reflection signalplot 300 is in relation to a vertical axis 304 representing voltage anda horizontal axis 308 representing time. The plot 300 is generated byprocessing in accord with the invention in response to sending asequence signal on the channel, receiving the reflection signal, andthen processing the reflection signal. The processed reflection signal300 reveals peaks or points of reflections at a time 312, 316, 320, and324. The peak at time 312 corresponds to the impedance mismatch createdby the hybrid 204. In some embodiments, the peak at time 312, caused bythe hybrid, is of significantly greater magnitude than the other peaks.

The peak at time 316 corresponds to the impedance mismatch created bythe anomaly 228 (FIG. 2A) caused by the connection of conductor 212 tochannel 208. The peak at time 320 corresponds to the impedance mismatchcreated by the line anomaly 220. The peak at time 324 corresponds to theimpedance mismatch created by the line termination 216. Based on thetime between pulses and the rate of propagation of a signal through themedium of the channel, the location of the anomalies or other impedancemismatches may be determined. By way of example, the propagation speedfor category 3 twisted pair cable is about two-thirds the speed oflight.

If the anomaly causes an impedance mismatch resulting in a decreasedimpedance, the reflection will have opposite polarity relative to theincident signal. If the anomaly causes an increase in impedance, thereflection will have the same polarity as the incident signal. Thus, anopen circuit will produce a positive return while a short circuit willproduce a negative return. In this manner the invention also providesinformation regarding the type of line anomaly. Other discontinuitiesmay also be mapped.

Sequence Signals

In one configuration, the invention comprises use of periodic sequencesfor channel analysis. In one configuration, the invention comprises useof any sequence with good autocorrelation properties. Theautocorrelation function of a sequence represented in continuous time,C(t), is given by: R(t) = ∫_(−∞)^(+∞)C(τ)C(τ + t)  𝕕τOne autocorrelation property is the Dirac delta function where R(t)equals infinity when t equals zero and R(t) equals zero for all othervalues of t. For practical finite sequences this can not be achieved.Therefore, with R(t) scaled such that the maximum value of R(t) equalsone, we define good autocorrelation properties as1−a≦R(t)≦1 for −p≦t≦+pe≦R(t)≦+e for t≦−(p+d), t≧(p+d)where a and e are small percentages of one, p is a percentage of thesequence symbol period and d is a small percentage of p. Bounded bythese requirements the values of a, e, p and d may be selected toprovide the desired sequence signal.

In one embodiment, the value ‘e’ is directly related to the period ofthe M-sequence. If ‘e’ is too large, the correlation process will resultin a side lobe. Hence, a small ‘e’ is desired but not required. In oneembodiment, ‘e’ is between 15% and 40% of one. In a more preferredembodiment, ‘e’ is between 5% and 15% of one. In a most preferredembodiment, ‘e’ is less than 5% of one. Similarly, in an embodiment, ‘d’is between about 15% and 45% of one. In a more preferred embodiment, ‘d’is between about 5% and 15% of one. In a most preferred embodiment, ‘d’is less than about 5% of one. It is preferred to reduce ‘p’, however, itis contemplated that various values of ‘p’ between zero and sequencesymbol period. Thus, it may range from zero to one. The value ‘a’influences width of the autocorrelation function. In some embodiments, anarrow impulse is desired. In one embodiment, ‘a’ is between about 15%and about 45% of one. In a more preferred embodiment, ‘a’ is betweenabout 5% and about 15% of one. In a most preferred embodiment, ‘a’ isless than about 5% of one.

In one embodiment, a requirement on R(t) in the transition regiondefined by d is that the function be reasonably smooth and decreasing.Therefore, use of sequences with good autocorrelation properties closelyapproximating an impulse can quickly and accurately provide the desiredreflection response information.

The autocorrelation function of a sequence represented in discrete time,C(n), is given by${{R(n)} = {\sum\limits_{k = {- \infty}}^{\infty}\quad{{C(k)}{C\left( {k + n} \right)}}}}\quad$One autocorrelation property in this case is the Kronecker deltafunction where R(n) equals one when n equals zero and R(n) equals zerofor all other integer values of n. Again, for practical finite sequencesthis function cannot be achieved. So, with R(n) scaled such that R(O)equals one, we define good autocorrelation properties asR(n)=1 for n=0−e≦R(n)≦+e for n≠0where e is a small percentage of one. In one embodiment, the e isdirectly related to the period of the M-sequence. If e is too large, thecorrelation process will result in a side lobe. Hence, a small e isdesired but not required. In an embodiment, ‘e’ is between 15% and 45%of one. In a more preferred embodiment, ‘e’ is between 5% and 15% ofone. In a most preferred embodiment, ‘e’ is less than 5% of one.

One example of a sequence well suited to be a line probing signalcomprises maximal length sequences (hereinafter M-sequences).M-sequences can be defined as a positive integer with no internalperiodicity. An M-sequence can be defined by the following equation:G(X)=g _(m) X ^(m) +g _(m−1) X ^(m−1) +g _(m−2) X ^(m−2) + . . . +g ₂ X² g ₁ X+g ₀whose coefficients are binary and where each arithmetic operation isperformed modulo two. When constructed as an M-sequence, the length orperiod of the sequence is defined as 2^(m)−1.

Sequences are desirable signals for numerous reasons. One reason is thatsequences can be generated by binary logic circuits, such as a scrambleror linear feedback shift register. Another desirable aspect of sequencesis that they may be generated at very high speed because of the type oflogic utilized to generate the sequence. Standard flip-flop andcombinational type logic may be used to generate these types ofsequences. Yet another desirable aspect of sequences, and M-sequences inparticular, is that these sequences possess good autocorrelationproperties that may be processed to closely approximate an impulse at apoint of correlation.

Sequences as contemplated by the invention may be implemented or createdin various ways. One method of M-sequence generation comprises use oflinear feedback shift registers. One example linear feedback shiftregister configuration comprises a Fibonacci implementation consistingof a shift register where a binary weighted modulo 2 sum of the taps isfed back to the input. Another example implementation comprises a Galoisimplementation consisting of a shift register, the contents of which aremodified at every step by a binary weighted value of the output stage.

This describes several particular types of sequence signals and isprovided for purposes of providing an enabling disclosure for at leastone class of sequences, however, the above description should not in anyway limit the invention. Any type of sequence may be utilized to obtainthe advantages over the prior as described herein.

FIG. 3 illustrates a plot of an example sequence transmission pattern.It is contemplated that the sequence signal may be repeatedly generatedand transmitted on the line in any various sequence. The sequenceincludes sequence period 406. Any number of sequence repetitions 410 maybe combined. A silence period 414 may also be provided after a sequencerepetition 410. The sequence repetition may comprise any number ofsequences 402. A silence period 414 may optionally be provided betweencertain sequences. An iteration period 420 comprises a repeating groupof sequences repetitions 410 and an optional silence period 414. In oneembodiment, two or more sequence repetitions 410 or sequences 406 aretransmitted in a row. Other combinations than the iteration period 420shown in FIG. 3 are contemplated. FIG. 3 is provided for purposes ofunderstanding and providing terminology to aid in understanding.

EXAMPLE EMBODIMENT

FIG. 4 illustrates a more detailed block diagram of an exampleembodiment of one configuration of the invention. Broadly, the elementsof FIG. 4 includes a transmit module 400 and a receive module 404.Connecting the transmit module 400 and the receive module 404 is a lineinterface 408 and other possible logic and lines (not shown). The lineinterface 408 connects the transmit module 400 and the receive module404 to a communication channel 412. The line interface 412 includesapparatus to separate or filter the transmitted signal from the receivedsignal and attempts to impedance match the transmit module 400 to thechannel 412 and the receive module 404 to the channel. In oneembodiment, the line interface 408 comprises a hybrid. The lineinterface 408 may also be configured to interface a single conductor ofthe transmit module 400 or the receive module 404 to twisted pairconductors. Although designed to reduce impedance mismatch, the lineinterface 412 often creates some mismatch and hence may create areflection during operation of the sequence time domain reflectometry asdescribed herein. This reflection may be referred to near-end echo.

In the example embodiment of the transmit module 400 shown in FIG. 4, asequence generator 420 connects to a PAM mapping module 422. Thesequence generator 420 generates a sequence signal. The output of thePAM mapping module connects to one or more transmit filters 424. Thetransmit filters 424 provide the sequence signals to a digital to analogconverter 426 and the output of the analog to digital converter connectsto the line interface 408.

With regard to the receive module, the line interface is configured toreceive and direct any reflection signals to an analog to digitalconverter 440. The output of the analog to digital converter 440connects to one or more receive filters 442 and the output of thereceive filters connects to a sequence correlator 446. The output of thesequence correlator 446 connects to a calibration and artifact reductionmodule 448, which in turn connect to an analysis module 450.

Transmit Module

The function of each element is now briefly described with more emphasison the elements that are of greater importance to the operation of theinvention and which may not be as well known. The sequence generator 420comprises any apparatus or system configured to generate a sequencesignal for transmission over the channel 412. In one embodiment thesequence generator 420 comprises at least partly software. In oneembodiment the sequence generator creates a maximal length sequence(M-sequence). In another embodiment the sequence generator creates aBarker Code type sequence. In yet another embodiment, the sequencegenerator creates a Kasami type sequence. In the embodiment shown inFIG. 4 having a sequence correlator 446, it is desirable for thesequence to have good autocorrelation or cross correlation properties.

In one embodiment, the sequence generator 420 is embodied in a scramblerto generate a pseudorandom bit pattern or sequence in an attempt tooutput a data stream without long sequences of constant voltage values.Various different embodiments exist for generating a sequence signal.

FIG. 5 illustrates a block diagram of an example embodiment of asequence generator configured using a linear feedback shift register orscrambler type implementation. An input 500 connects to a summing unit504. All arithmetic operations may be performed in a modulo-2 fashion.The summing unit 504 has an output connected to an output line 508 and adelay register 510A. The output of the delay register 510A connects to amultiplier 514A, having a multiplier set to C₁, and to another delayregister 510B. The output of delay register 510B connects to N number ofother delay registers and multipliers until connecting to a delayregister 510C and to a multiplier C_(N−1). The output of delay register510C connects to a multiplier 514C that has a multiplier C_(N). Thiscreates an Nth order generator due to the N memory elements or delayregisters 510. This thus generates an output based on the content of theregisters, also known as the state of the scrambler. Thus, the totalnumber of different possible states of the generator is 2^(N).

In one example method of operation, a continuous sequence of logic value1's is provided to the input 500. The state of each register may beselectively loaded with a logical one or a logical zero based on thedesired sequence to be generated. When provided with a string of logicsone values, the generator outputs a unique string, or sequence, of 1'sor 0's based on the values of the registers 510. In one embodiment, thevalues loaded into the registers are selected to form a primitivepolynomial known to generate a maximal length sequence (M-sequence). Thesequence will repeat through the 2^(N)−1 non-zero states.

FIG. 6 illustrates an alternative embodiment of a sequence generator.The embodiment shown in FIG. 6 comprises a tapped delay lineconfiguration designed to generate a sequence for use with the systemsdescribed herein. As shown in FIG. 6, an input 604 connects to a delayregister 608 that is configured to receive and delay for a clock cycleor other period the received value. The input 604 also connects to amultiplier 612A having a multiplier value M₀. All arithmetic operationsin this embodiment may be performed in the traditional fashion, that is,not modulo-2. The output of the multiplier 612A connects to a summingjunction 624.

The output of the register 608 connects to multiplier 612B having amultiplier value M₁. The output of the multiplier 612B connects to thesumming junction 624 to add the output of the multiplier 612B and themultiplier 612A. The output of the register 608 also connects to aregister 616, the output of which connects to multiplier 612C. Theoutput of the multiplier 612C connects to summing junction 636, whichalso receives the output of summing junction 624. The tap delayed line600 continues in this configuration until connecting to a register 632that has an output connected to a multiplier 612D with a multiplierfactor M₂ ^(N) ⁻¹. The output of multiplier 612D connects to a summingjunction 644 that also receives the output of the previous summingjunction.

This configuration is 2^(N)−1 long with the elements of the tapped delayline controlling the sequence generated. Specifically, the coefficientsof the tapped delay line are the sample values of the desired sequencesignal. An input of a pulse followed by zero-valued samples to thetapped delay line propagates through the tapped delay line and as thepulse propagates through the line, it encounters the multiplier valuesof the multipliers 612. The multiplier value will propagate to theoutput since all other coefficients are multiplied by zeros. In oneembodiment, the multiplier values may comprise a logical 1 or a logical0. The multipliers 612 each pass a logical 1 to its associated summingjunction or pass a logical 0 to its associated summing junction. Hence,a sequence signal is output with values controlled by the values of themultipliers 612. In a variation of this embodiment, the values of themultipliers may be selected as other than 1's or 0's to thereby generatea mapping as is performed by the mapping module 422 shown in FIG. 4. Insuch a variation, the mapping module 422 can be eliminated.

Yet another embodiment of the sequence generator comprises a tablelook-up system. In a table look-up system, a sequence signal is storedin memory or a look-up table and recalled u sing a software interface.Hence, upon request of a particular sequence signal, the sequencegenerator 420 performs a table look-up, recalls the desired sequencesignal from memory, and provides the sequence to the other systems ofthe transmit module 400. Any number or variation of sequences signalsmay be stored or retrieved.

Returning now to FIG. 4, the signal mapper 422 transforms the digitaloutput of the sequence generator to any various signal levels thatrepresent bit values. For example, four bits of digital data may berepresented as 16 PAM, i.e. any of 16 different numerical values. The 16different values may be represented on a scale of minus one to seveneighths in increments of ⅛. The signal may be scaled by an amplifier toyield a desired transmit power. In one embodiment the signal mapper 422comprises a table look-up device or process that translates the binaryinput to a numeric output.

The transmit filter 424 is configured to manipulate the output data toadhere to desired or required spectral requirements. For example,frequency filtering may occur to improve system performance by tailoringthe frequency content of the output or it may simply be mandated by FCCor a standards organization. It may be desired to attenuate out-of-bandenergy while also minimally effecting in-band energy. The embodimentshown in FIG. 4 implements spectral shaping with a digital filter. Ananalog filter may serve to reject images of the digital processing.Another embodiment eliminates any digital transmit filter. In such anembodiment, the spectral shaping is provided by the analog filter.

The digital to analog converter 426 is generally understood to convert adigital signal to an analog signal. In the embodiment shown, thetransmission on the line occurs in an analog format.

Although not shown, an analog filter may also be included just prior tothe line interface 408 in the transmit module 400 to perform finalfiltering of the analog waveform to spectrally prepare the signal fortransmission over the channel 412. The analog filter may operatesimilarly to the transmit filter 424 but in the analog domain.

EXAMPLE SEQUENCES

In one configuration, the sequence generator 420 or other device withsimilar capabilities generates a sequence defined by varying thepolynomial of the sequence generator to provide different sequencesignals. In another configuration, the polynomial is selected tomaximize the period of the sequence, such as to create an M-sequence. Asdescribed above, the period of a length-maximized sequence is defined as2^(m)−1 where m is the number of stages of shift registers used togenerate the sequence.

By varying the number of stages m, the period is controlled. Variousadvantages may be gained by varying the period of the sequence. Forexample, one advantage of increasing the period of the sequence whenused according to the invention for sequence time domain reflectometryis in mitigating the effects of correlated additive noise such ascrosstalk. In the correlator, the noise component is decorrelated whichspreads the noise across all frequencies thus reducing the amount ofnoise in the frequency band of interest. This improves the accuracy ofthe channel analysis. Another advantage of increasing the period of thesequence is that the system can provide a more complete response andlonger channels may be analyzed. Yet another advantage of increasing theperiod of the sequence is that the reflection analysis is based on moretones with finer frequency spacing. Increasing the sequence period doesnot decrease the temporal resolution of the analysis. The temporalresolution is determined by the duration of one element of the sequencenot the total length of the sequence.

An advantage of a shorter period generated by using a smaller m value isthat the sequence may be generated and analyzed more rapidly. Thisspeeds the process. Another advantage of shorter period sequences is alowering of the computational complexity in the receiver.

Although numerous specific sequences are provided below, it iscontemplated that any type sequence may be used. The text Introductionto Spread Spectrum Communications written by Peterson, Ziemer and Borth,(Prentice Hall, 1995), which is incorporated herein in its entirety,provides a discussion on different sequences and in particular differenttypes of M-sequences. Table 3-5, from the above-referenced text,provides a list of primitive polynomials that may be used to generatethe sequence. Any sequence period may be selected. Other sequencesignals that are contemplated for use with the invention, than thoselisted, also exist.

In general, numerous M-sequences exist with periods depending on thenumber of stages in the shift register. There is at least one M-sequencefor every integer greater than one where this integer represents thenumber of stages of the shift register. If more than one M-sequenceexists for a given number of stages then the sequences are distinguishedby the non-zero taps of the shift register. This is designated by thepolynomial representation. In one embodiment of the invention, asequence having a period of 31 is generated by a modem or othercommunication device, or test equipment, which may be located at anypoint of a communication channel. One polynomial defined by a period of31 is:

 s(n)=s(n−2)⊕s(n−5)⊕ƒ(n)

where f(n) is the logical ones input to the sequence generator, s(n−k)is the tap point after the k-th delay element in the sequence generatorand ⊕ is modulo-2 addition.

Another example polynomial that may be generated by a communicationterminal and is defined by a period equal to 63 is:s(n)=s(n−1)⊕{dot over (s)}(n−6)⊕ƒ(n)

Another example polynomial that may be generated by a communicationterminal and is defined by a period equal to 127 is:s(n)=s(n−3)⊕s(n−7)⊕ƒ(n)

Another example polynomial that may be generated by a communicationterminal and is defined by a period equal to 255 is:s(n)=s(n−2)⊕s(n−3)⊕s(n−4)⊕s(n−8)⊕ƒ(n)In another embodiment of the invention, a sequence having a period of 31may be generated by a communication terminal and adopted for use as asequence signal. One polynomial defined by a period of 31 is: s(n)=s(n−3)⊕s(n−5)⊕ƒ(n)where f(n) is the logical ones input to a sequence generator, s(n−k) isthe tap point after the k-th delay element in the sequence generator and⊕ is modulo-2 addition.

Another example polynomial that may be generated by a communicationterminal and is defined by a period equal to 63 is:s(n)=s(n−5)⊕s(n−6)⊕ƒ(n)

Another example polynomial that may be generated by a communicationterminal and is defined by a period equal to 127 is:s(n)=s(n−4)⊕s(n−7)⊕ƒ(n)

Another example polynomial that may be generated by a communicationterminal and is defined by a period equal to 255 is:s(n)=s(n−4)⊕s(n−5)⊕s(n−6)⊕s(n−8)⊕ƒ(n)

The term communication terminal is defined to mean any configuration ofsoftware or hardware configured to facilitate or perform communicationor generate a signal or sequence. In another embodiment the termcommunication terminal is defined to mean a piece of test equipment.This includes a modem, scrambler, sequence generator or other similardevice, or a separate, stand-alone device.

Using the sequence signals, generated by the sequence generator,scrambler, or any other device capable of generating a correspondingsequence signal for time domain reflectometry provides advantages overthe prior art signal of a single high power pulse. One such advantagecomprises the ability to implement the sequence time domainreflectometry in an integrated circuit, such as within a communicationdevice.

Receive Module

The receive module 404 includes the analog to digital converter totransform the received reflection signal from the analog domain to thedigital domain. An amplifier (not shown) may be placed between the lineinterface 408 and the analog to digital converter 440 to amplify thepossibly weak reflection signal from the channel 412. In one embodiment,the analog to digital converter 440 comprises a fourteen bit converter.Increasing the precision of the converter improves the dynamic range ofthe receive allowing smaller magnitude returns to be detected, such asthose from a very long transmission line.

The receiver filters 442 comprise standard filters such as high and lowpass filters to eliminate unwanted frequency components that are outsideof the frequency band of the reflection signal. Any type of digitalfiltering may be performed by the filters 442. In addition, analogfilters (not shown) may be located prior to the analog to digitalconverter 440 as necessary to filter the reflection signals receivedfrom the line interface 408 prior to conversion into the digital domain.

The sequence correlator 446, which receives the output of the receiverfilters 442, comprises a configuration of hardware, software, orcombination thereof, that is configured to correlate the reflectionsequence signal with a copy or duplicate of an original sequence signalthat was generated by the sequence generator 420. Although not shown,the sequence correlator 446 may communicate or connect to the sequencegenerator 420. In one embodiment, the correlation comprises crosscorrelation. Mathematically, in one embodiment, a crosscorrelator isrealizing the following function:${h(n)} = {\sum\limits_{k}{{C(k)}{X\left( {k + n} \right)}}}$where X(n) is the sum of the transmitted sequence C(n) plus any additivenoise and crosstalk. In one embodiment the correlator 446 is embodiedusing a sliding tapped delay line. There are numerous ways to implementthe correlator 446 and this is but one example embodiment. Thecorrelator 446 may be embodied in hardware, or software, or acombination of the two. Indeed, it is contemplated that an analogimplementation of the correlator maybe preferred particularly in highrate applications. In this implementation analog to digital converter440 maybe omitted. In the sliding tapped delay line method the taps areC(n).

One example embodiment of a cross correlation device is shown in FIG. 7.FIG. 7 illustrates block diagram of a correlation unit configured tocorrelate a received signal with a signal C(n). An input 704 connects toa multiplier 708. A second input 712 provides a second signal to themultiplier 708. The output of the correlator connects to a summingjunction 718, which has an output 720.

The received reflection signal is provided on input 704 to themultiplier unit 708 while a sequence signal C(n), that is generallyidentical to the sequence signal transmitted on the channel, is providedon the second input 712. These sequence signals are multiplied togetheron a value by value basis over time. The output of the multiplier 708 issummed, over time, in the summing junction 718 and provided on theoutput 720. The correlation system provides an output signal with a peakat the point when the signals align, i.e. correlate. A noticeable peakat the point of correlation indicates a sequence with good correlationproperties.

The accumulator or summing junction 718 comprises a device configured togenerate a running summation of the received signals. In general, theoutput of the summing junction 718 is generally similar to a first orderapproximation of an integral over the period of time that the systemoperates. Thus, the summing junction 718, upon receipt of a number,stores the number. Then, upon receipt of another number, the summingjunction 718 adds the first number to the second number and stores theresult. The process continues in this manner. In one embodiment, thesumming junction 718 comprises one or more registers to store theaccumulating result. The output of the correlation process is anestimate of the impulse response of the channel. This is a time domainsignal.

Another example embodiment of the cross correlation is based onfrequency domain processing. The cross correlation can be implemented inthe frequency domain by multiplying together the frequency domainrepresentation of the received signal and the reference signal. Thereference signal may be the discrete Fourier transform (DFT) of thetransmit sequence, inverted in time. When periodic sequences are used,the frequency domain representation can be constructed by using a DFT ofthe same length as the period of the signal. If the receive signalconsists of multiple periods, then t he noise characteristics of thecorrelated signal can be improved by appropriately summing up multipleperiods, either before or after taking the DFT of the received signal.For non-periodic signals or signals with long periods, it may beappropriate to compute the cross correlation in the frequency domainusing the overlap-add or overlap-save methods. If the cross correlationis computed in the frequency domain, it may be appropriate to convert itback to the time domain for further time domain processing.

Returning to FIG. 4, as a result of correlating the reflection signalwith the sequence signal as originally transmitted on the channel 412,the output of the correlator 446 provides a signal that may generallyresembles the plot shown in FIG. 2B, although unwanted components may bepresent. When an echo signal is aligned with the sequence signal in thecorrelator, a peak occurs at the output. In this manner, the output ofthe correlator provides an indication of when the received signalcontains an echo or point of reflection. These points correspond toanomalies in the line.

Advantages Regarding Noise

Another advantage of the correlation processing that occurs from use ofa sequence signal having good correlation properties is with regard tonoise. The invention sends a plurality of bits in the form of a sequencesignal and then monitors for the received reflection signal, which isalso a plurality of reflected bits in the form the sequence signal. Eachanomaly generates a sequence of reflections. As a result of thespreading of the signal over a plurality of bits in the sequence, thereceived noise, such as random noise from static, interference orcrosstalk, is spread over the length of the reflection sequence.

FIGS. 8A and 8B, which illustrates example plots of a sequence signaland the effect of correlation, are helpful in describing the advantagesgained by the invention with regard to noise. FIG. 8A illustrates a plotof a sequence signal 800 in relation to a vertical axis 802 representingmagnitude and a horizontal axis 804 representing frequency. Anundesirable noise component 810 resides between frequencies f₁ and f₂.If a single pulse signal is transmitted, the noise that will be receivedwith the reflection signal will disrupt analysis.

In reference to FIG. 8B showing a plot of the correlated signal 820 andthe noise 822 that is part of the correlated signal after correlation inrelation to magnitude on the vertical axis 802 and time on thehorizontal axis 830. During the correlation process, the originalsequence and the reflection sequence only correlate at the point ofalignment, that is between times T₁ and T₂. Thus, noise on thereflection signal is disbursed over the time period of the correlationprocess. Correlation serves as a summation only at the point ofcorrelation thereby reducing the effects of the noise. Hence, noise is asmaller portion 822 of the correlated signal because the noise isspread. Thus, the invention reduces the effect of noise on the line.

Another advantage of the invention is that it allows for thetransmission of a lower power signal over the channel. Use of a lowpower signal eliminates interference, such as from crosstalk, with otheradjacent lines, such as other pairs in the binder. Use of a low powersignal provides the further advantage of enablement using an integratedcircuit, such as built into a modem. This eliminates the requirement forthe sequence time domain reflectometry system to be built into aseparate piece of test equipment that is constructed to enablegeneration and transmission of a high power pulse.

It is contemplated that the power level of the sequence may be of anymagnitude. In one embodiment the power level may be constrained byapplicable standards such as the ITU G.shdsl or ANSI HDSL2 standards.This may be implemented by use of transmit filtering which conforms tothe power spectral density constraints imposed by those standards. Sincethe sequence signal may be a valid data signal, it may conform to thestandard specifications if the same transmit filtering is employed. Thisis not true in general for single pulse systems, which use anundesirable high power pulse.

In one embodiment the peak voltage of the sequence signal is less than 6volts. In another embodiment, the peak voltage of the sequence signal isbetween 6 volts and 18 volts. In yet another embodiment, the peakvoltage of the sequence signal is higher than 18 volts. This are butexample ranges. Any peak voltage or power level may be selected.

Returning to FIG. 4, the calibration and artifact reduction module 448may comprise software or hardware configured to manipulate or eliminateportions of the reflection signal. In one embodiment, the calibrationand artifact reduction module reduces correlation artifacts. In oneembodiment the calibration and artifact reduction module 448 comprisesan interface to memory configured to recall one or more differentsignals or template signals. The signals or templates may comprisestored, calculated, recorded, or estimated behavior of one or morecomponents or interfaces in the system in relation to a sequence signal.By subtracting the stored, calculated, recorded, or estimated behaviorfrom the received reflection signal, unwanted or undesired portions ofthe reflection signal may be eliminated or reduced. This process isreferred to herein as calibration. The template, to be subtracted fromthe received reflection sequences to thereby modify the reflectionsequences, may be stored in memory, generated, or obtained bymanipulation of stored data to obtain the desired signal. In oneembodiment, the stored template is already correlated. In anotherembodiment, the stored template is not correlated until after beingrecalled from memory.

In one embodiment, the configuration of the line interface 408 may besuch as to create a reflection such as near-end echo. Because the lineinterface 408 is close to the transmitter and receiver, the resultingnear-end echo will have a large magnitude in relation to the reflectioncreated from distant line anomalies. Such a disproportional signal maydisrupt analysis of the reflections at issue and hence it may bedesirable to reduce or eliminate this signal.

In one particular embodiment, the calibration and artifact reductionmodule 448 is configured to eliminate the reflection created by the lineinterface 408. In such an embodiment, one or more sample reflectionsignals, also referred to as templates, are stored in memory or meansprovided to generate or recall these template signals. After executionof the channel analysis and a reflection signal being received by thecalibration and artifact reduction module 448, the module 448 recallsthe template from memory or generates the template and subtracts thetemplate from the received reflection signal. This removes or reducesthe effects of the line interface to thus provide greater accuracyduring subsequent processing. Prior to subtraction, the template isproperly aligned with the reflection signal.

The analysis module 450 receives the signal from the calibration andartifact reduction module 448. In one embodiment, the analysis module450 is embodied in software, stored on computer readable media andconfigured for execution by a processor or other software executiondevice. In one embodiment, the analysis comprises synchronization of thereflection signal to a time of transmission over the line. Based on thesynchronization, the time between the start of the transmission and thereceipt of each reflection peak can be calculated. Timers, a timingmodule, or counters in conjunction with peak detectors may be used todetermine the time between sequence transmission and peak detection. Thefollowing equation may be used to calculate the distance to the lineanomaly from the line interface. Assuming a rate of propagation of about⅔ λ for twisted pair, where λ is the speed of light in units/second,then:${{distance}({units})} = {\left( {\frac{2}{3}\lambda} \right)\frac{\times {time\_ until}{\_ reflection}{\_ peak}}{2}}$This accurately provides the distance to the anomaly and thus allowsservice technicians to quickly locate and remedy or remove the anomaly.Various different mediums have different rates of propagation. Inaddition, the number and severity of the effect of the anomalies may bedetermined and a decision made regarding whether to repair or abandonthe line. Of course, this is but one possible method of analysis.

FIG. 9 illustrates a plot of an example sequence signal 950. Signalamplitude 952 is represented on the vertical axis and time 954 isrepresented on the horizontal axis. The example sequence is provided forpurposes of discussion only. Other sequences are contemplated.

Alternative Embodiment

FIG. 10 illustrates an alternative embodiment of the invention. Ascompared to FIG. 4, similar elements are identified with identicalreference numerals. As shown, output from the receiver filters 442connects to an adaptive prediction filter 908. The output of the filter908 connects to a calibration module 904, which in turn has an outputconnected to the analysis module 450. The adaptive prediction filter 908may comprises any type system configured to generate coefficients orother representative signals or values that portray the reflectionresponse of the channel 412. The adaptive prediction filter 908 comparesa reflection received from the channel to its own output and dynamicallyadjusts its internal coefficients or values to generate a signal that isgenerally identical to the reflection channel. This may occur via theuse a feedback link. The internal coefficients or values of the adaptiveprediction filter thus define the channel and can be analyzed todetermine the location of anomalies in or on the line.

FIG. 11 illustrates a block diagram of a representative configuration ofa prediction filter configured to match the channel response. An input1002 connects to the prediction filter 1004 and to the channel 1008. Thereflection output of the channel 1008 is represented by the reflectionsignal r(t) 1016. The output of the prediction filter 1004 comprises anadapted signal r′(t) 1012 and feeds into a summing junction 1020 as anegative input. The reflection signal r(t) 1016 also connects to thesumming junction 1020. Thus, the predictive filter output r′(t) 1012 issubtracted from the reflection signal r(t) 1016. The resulting output ofthe summing junction 1020 comprises an error signal 1024 representingthe difference between the prediction filter output 1012 and the actualreflection signal 1016. The error signal 1024 feeds back into theprediction filter 1004. If the error signal is not zero or about zero,the prediction filter 1004 adjusts its coefficients or internal valuesto force the error signal 1024 to zero. When the error signal 1024 iszero or about zero, then the coefficients or values of the predictionfilter represent the reflection channel. The sequence defined by thefilter coefficients is an estimate of the impulse response of the echochannel, which is comprised of the near-end echo path and thetransmission paths to and from each anomaly, which causes a reflection.Thus, it is an estimate of the correlated sequence signal and the sameprocessing analysis can be employed that was applied in the embodimentof FIG. 4.

One example embodiment of an adaptive predictive response filtercomprises a tapped delay line configuration as shown in FIG. 6. Themultiplier values M are the values of interest in the prediction filter1012. This may be implemented as a finite impulse response filter. TheElectronic Handbook, edited by Jerry C. Whitaker from CRC Press, Inc.,1996, which is incorporated in its entirety herein, contains adiscussion of digital and adaptive filters at page 749-772. It iscontemplated that a direct form structure or a module form structure maybe used.

Although the tapped delay line type system is described, it iscontemplated that any echo cancellation type system may be adopted foruse. Any system configured to generate a signal that is generally thesame as a reflection signal received over the channel in response to thesending of a sequence signal may provide the necessary information toperform time domain reflectometry analysis of the channel. Once thesystem is configured in this manner, a single impulse followed by zerosapplied at the input of the prediction filter will produce the impulseresponse of the reflection channel at the output of the predictionfilter. It may be desirable to simply the utilized coefficient values ofthe prediction filter instead of inputting a pulse followed by zeros.

Operation

FIG. 12 illustrates an example method of operation of one embodiment ofthe invention. This is but one example embodiment. It is contemplatedthat other methods of operation are possible and within the scope of theinvention as define by the claims. At a step 1202, the sequence timedomain reflectometry system (hereinafter system) generates a sequencesignal. The sequence signal may comprise an M-sequence or any other typeof sequence. In one embodiment, the sequence comprises a sequence withgood autocorrelation properties. At a step 1204, the operation performssignal mapping to assign the sequence signal to one of several differentvalues. At a step 1206, the system filters the signal to remove unwantedcomponents. At a step 1210, the system converts the digital sequencesignal to an analog format. At a step 1212, the system transmits thesequence signal over a communication channel. It is understood thattransmission of a signal over a channel will generate reflections atpoints of impedance mismatch i.e., line anomalies.

At a step 1214 the system monitors for and receives any reflectionsignals generated by the transmission of step 1212. The reflectionsignal may be defined as the overall signal(s) received during a periodof time after the transmission of the original sequence signal over thechannel. Thus, the reflection signal may actually comprise severalperiods of silence and several individual echoes created by the sequencesignal encountering impedance mismatches or other anomalies as ittravels down the channel. During receipt, the reflection is converted toa digital format at a step 1216. The signal may be stored or processingmay continue at step 1220 by filtering the reflection signal to removesignals at unwanted frequencies. At step 1222, the system correlates thereflection signal with the original sequence signal. The correlationreveals the location of peaks within the reflection signal. Consideredin different terminology, the channel is monitored after thetransmission of the sequence signal for a period of time sufficient forany reflections generated by the transmission to be recorded by themonitoring. The received signals during this period of time areconverted to the digital domain and stored or processed. Correlationoccurs at step 1220 between the original sequence signal and any signalsrecorded during the monitoring period of step 1214. Peaks in thecorrelated signal occur at the points of time in the received signalwhen a reflection was received.

At a step 1224, the system synchronizes in time the correlator outputwith the start of the sequence. This allows for an identification of atime, in relation to the start of the sequence signal transmission orother reference point, at which peaks or reflections occur. The peak ofthe near-end echo may serve as the reference time point. At a step 1226,the system removes unwanted artifacts or disruptive reflections. Oneexample of a disruptive artifact is near-end echo created by hybrid.Thereafter, at a step 1230, the system analyzes the correlatedreflection signal to determine the location of line anomalies. This mayoccur by processing the reflection signal to determine the timedifference between the start of the sequence signal transmission and thepeak in the correlator of the reflection signal. The time difference ismultiplied by the rate of propagation of the signal through the channel.This provides the combined distance to and from the line anomaly usingthe transmitter or line interface as a reference point. The distancevalue may be divided by two to arrive at a distance to the anomaly.

FIG. 13 illustrates a n example method of sequence generation. Numerousdifferent methods of sequence generation are possible. The embodimentshown in FIG. 13 comprises generation by use of a linear feedback shiftregister (LFSR). At a step 1302, the sequence generation operationinitiates the channel analysis process. Next, at step 1306, a specificsequence is designated for use. One characteristic of a specifiedsequence is its period. At step 1310, the operation preloads registersof the linear feedback shift register with values necessary to realizethe specified sequence. At a step 1314, the operation begins inputting aconstant sequence of logical 1's into the sequence generator.Thereafter, at a step 1318, the operation processes the series oflogical 1's through the sequence generator to create the specifiedsequence signal.

FIG. 14 illustrates an alternative method of sequence generation such asmight be implemented for use with a table look-up method. At a step1402, the channel analysis process is initiated. Thereafter at a step1406, the operation specifies a sequence for generation. Once thedesired sequence is specified at a step 1410, the system obtains or isprovided a memory address for the sequence data. Once the location inmemory or the look-up table is provided or obtained, the system beginsoutputting the data items of the sequence. This occurs at step 1414. Theoperation then progresses to a step 1418 where the system queries todetermine if there are additional data items remaining in the sequence.If additional data items exist, then the operation returns to step 1414and an additional data item is output. If at step 1418 there are no moreadditional data items in the sequence to be output, then the operationprogresses to a step 1422 to indicate that the sequence is complete andthat the receiver aspects of the sequence time domain reflectometryshould begin monitoring for reflection signals.

It should be noted that in the methods of FIGS. 13 and 14, the sequencemay be generated and transmitted once, generated numerous times andsequentially transmitted numerous times, or generated at transmitted insome pattern with a period of silence between one or more sequencetransmissions.

FIG. 15 illustrates an operational flow diagram of an example method ofcorrelation and processing of a reflection signal. At a step 1502, thereflection signal is provided to the correlator. In addition, at a step1506, the operation also provides the original sequence signal to thecorrelator. In one embodiment, this comprises loading the coefficientsof the generator polynomial of the original sequence as coefficients ina scrambler to generate the same sequence as was originally transmittedover the channel. Next, at step 1510 the correlator multiplies theoriginal sequence, on a point-by-point basis with the reflection signal.At step 1514, a correlator creates a running summation of the results ofthe multiplication of a step 1510. Next, at a step 1518, the operationstores the correlator output as the correlated reflection signal.

Next, at a step 1522, the system initiates an artifact reductionroutine. The artifact reduction routing is discussed below in greaterdetail in conjunction with FIG. 16. After artifact reduction, thesystem, at a step 1526, receives an artifact free, correlated reflectionsignal.

At a step 1530, the system initiates a near-end echo reduction routine.FIG. 17 provides an operational flow diagram of an example method ofnear-end echo reduction. After near-end echo reduction, the systemreceives a correlated reflection signal generally absent of near-endecho and generally without correlation artifacts. This occurs at a step1534. At a step 1538, the operation initiates a time based processing todetermine the location of line anomalies. This process is described ingreater detail below in conjunction with FIG. 18.

FIG. 16 illustrates an operational flow diagram of an exemplary methodof artifact reduction. The term artifact as used herein is defined tomean unwanted signal components that are generated by the correlation ofthe original sequence signal and the reflection signal(s). In oneembodiment, a continuous stream of repeating sequences is not sent. Insuch an embodiment, one or more sequences are sent, followed by a periodof silence. Correlation of a non-continuous stream of sequences may leadto partial correlations and thereby generate artifacts before and afterthe peaks generated at the point of correlation. These artifacts may bereferred to as side lobes. The goal of artifact reduction is to removeor reduce the artifacts to thereby more clearly define points ofcorrelation.

In one embodiment shown in FIG. 16, at a step 1602, the operation storesan original correlated reflection signal. This signal will be used insubsequent processing. Next, at a step 1606, the artifact reductionmodule performs circular rotation on a copy of the original sequencesignal. Circular rotation comprises shifting of the values of thesequence by a shift constant k. The shift constant k determines thenumber of elements the sequences is rotated. By way of example, a shiftconstant of two shifts the original sequences defined by:{M₀, M₁, M₂, M₃, M₄, M₅ . . . M₂ ^(N) ⁻²}becomes:{M₂ ^(N) ⁻³, M₂ ^(N) ⁻², M₀, M₁, M₂, M₃, M₄, M₅ . . . }after a circular shift with a shift constant k=2.

Next, at a step 1610, the system transmits the rotated sequence signalover the channel and the system monitors for a reflection signal. Atstep 1614, the system receives the rotated reflection signal thatresults from the transmission of the rotated sequence signal. Afterreceiving and processing by the receiver, at a step 1618, the receivedrotated sequence signal is correlated with the rotated sequence signal.The rotated sequence signal is the signal that was transmitted to createthe rotated reflection.

After this correlation, there exists a correlated rotated signal createdfrom the transmission of the rotated sequence signal, receipt of arotated reflection, and correlation of the rotated reflection with therotated sequence signal. There also exists the original correlatedreflection signal created by the transmission of the sequence signal,receipt of its reflection, and correlation of the reflection with thesequence signal. At step 1622 these two signals, the correlatedreflection signal and the correlated rotated signal are combined causingthe unwanted artifacts to generally cancel out. In some instances notall artifacts will cancel, but will be significantly reduced. Thisprocess may be repeated as needed using different shift constants tofurther reduce the artifacts. As a result, peaks representing thereflection created by a line anomaly are more clearly noticed anddetectable. At a step 1626, the operation returns an artifact free,correlated reflection signal for further processing.

FIG. 17 illustrates an operational flow diagram of an example method ofnear-end echo reduction. One common source of near-end echo is the lineinterface, such as a hybrid. In many instances, the near-end echocreated by the line interface is of a greater power level than otherreflections caused by distance line anomalies. As a result, the highpower near-end echo may mask or drown out the weaker reflections frommore distance anomalies. Thus, it may be desirable to perform near-endecho reduction or removal.

At a step 1702, a near-end echo reduction module selects a template forthe near-end reduction processes. At step 1706, the template isretrieved from memory. The term template as used herein is defined tomean stored data that corresponds or relates to the behavior of the lineinterface or other source of undesirably large echo. In one embodiment,one or more templates that correspond to the behavior of differenthybrids are stored in memory for recall. In another embodiment, thenear-end echo reduction module transmits an example sequence, andmonitors the hybrid response and stores this response as the template.The template, as stored, may be correlated or uncorrelated.

At a step 1710, the system correlates the template signal that isrecalled from memory with the reflection signal. Correlating these twosignals creates a peak at the point where the two signals align. Thus,at step 1714, the operation selects the point in time when the twosignals correlate. Using the point in time identified as the point whenthe two signals correlate, the operation moves to a step 1718 and alignsthe template signal with the correlated reflection signal. At a step1722, the template is subtracted from the correlated reflection signalto remove the near-end echo. Thereafter, at step 1726, the operationreturns to processing as referenced in FIG. 15. This removes thenear-end echo. It is contemplated that in other embodiments templatesother than those corresponding to the reflection from the line interfacemay be stored and subtracted from the reflection signal. Thus, if otheraspects of the reflection signal are to be removed or reduced, themethod of FIG. 17 may be utilized.

FIG. 18 illustrates an example method of processing the sequence signalto determine the location of line anomalies. In one embodiment, thesequence time domain reflectometry system is built into a modem. In oneembodiment, the processing comprises a two-part process; alignment andtime measurement. At a step 1802, the processing operation receives thecorrelated reflection signal. In one embodiment at this stage, thecorrelated reflection signal has undergone correlation, near-end echoreduction, and artifact reduction. After receipt, the operation may timesynchronize the signal based on the information obtained during thenear-end echo reduction processes. In one embodiment, the peak ofnear-end echo is the beginning of the reflection signal because thenear-end echo occurs generally simultaneously with the start of thesequence transmission. Working from this basis, the time at which thepeak of the near-end echo occurs is taken to be the start of the signal.This occurs at step 1810. In one embodiment, this information isprovided from the near-end echo reduction module described inconjunction with FIG. 17.

At a step 1814, the peak in the near-end echo is assigned T₁ andreferenced as time=0. Thereafter, at a step 1818, the operationcalculates, in relation to T₁, the time at which the next peak in thereflection signal occurs. This is assigned time T₂. At step 1822, theprocessing subtracts T₂ from T₁ to determine the time it took betweenthe sequence signal start and the first reflection. This time isassigned T_(R1) for purposes of this discussion. Next, at step 1826, theprocess multiplies T_(R1) by the velocity of propagation for the signalthrough the medium of the channel. This calculation yields a distancevalue, which reveals the location of the first line anomaly. At a step1830, the operation repeats for the other peaks in the reflectionsignal.

FIG. 19 illustrates an operational flow chart for an example method ofoperation for sequence time domain reflectometry using echocancellation. This alternative embodiment is shown in FIG. 10. At a step1902, this embodiment receives the reflection signal at the lineinterface and thereafter, at a step 1906, performs receiver processingon the reflection signal to prepare the signal for further processing.Next, at a step 1910, the operation initializes an echo canceller byloading estimated coefficient values into the echo canceller. Thisprepares the echo canceller to receive an input and generate an output.In one embodiment, the stored estimated coefficients comprisecoefficients that are estimated to closely resemble the coefficientsthat will eventually be selected for the echo canceller. In oneembodiment, the echo canceller comprises a finite impulse responsefilter. In one embodiment, the echo canceller includes a Volterra seriesexpansion to model non-linear affects such as cable resistance,inductance and capacitance.

At a step 1914, the embodiment inputs the reflection signal into theecho canceller causing the echo canceller to generate an output based onthe input and the loaded coefficients. The operation progresses to astep 1918 whereby the output of the echo canceller is subtracted fromthe reflection signal and any error or difference between the signalsmeasured and fed back into the echo canceller. The error signal is thedifference between the echo canceller output, which is determined by thecoefficient values, and the reflection signal.

At a step 1922, the system determines if the error signal isapproximately equal to zero. A generally zero error signal or equivalentis desired. If the error signal is not zero, then the operation, at astep 1926, adjusts the coefficients of the echo canceller to cause theerror signal to approach zero. This processes continues until the errorsignal is generally zero. When, at step 1922, the error signal isgenerally zero, then the echo canceller coefficients are read at step1930, from the echo canceller. These coefficients, when considered as asequence, form an estimate of the impulse response of the reflectionchannel and can be used to determine the location of the line anomalies.The coefficients can be considered as the impulse response or a pulsefollowed by zeros may be fed into the echo canceller and the outputrecorded.

At step 1934, the operation may perform calibration to remove near-endecho or other unwanted signal components. In one embodiment this may beconsidered signal shaping. At a step 1940, the system calculates thetime between peaks of the impulse response of the reflection channel.Working from the time between pulses, processing occurs to calculate thedistance to line anomalies based on the time at which peaks occur in theimpulse response.

This is an exemplary method of operation of the alternative embodimentof sequence time domain reflectometry using echo cancellationtechniques. It is contemplated that other methods of processing may beadopted for use with the echo canceller embodiment. The scope of theclaims is not intended to be limited to this particular method ofoperation, but is intended to cover any method of sequence time domainreflectometry utilizing the coefficients of a prediction filter.

While various embodiments of the application have been described, itwill be apparent to those of ordinary skill in the art that many moreembodiments and implementations are possible that are within the scopeof this invention.

FIG. 20 illustrates a block diagram of an example embodiment of anoptical sequence time domain reflectometry system. The embodiment ofFIG. 20 is in an example environment of test equipment 2004. The testequipment 2004 may comprise any type of test equipment configured toperform testing or analysis on a communication channel. In theembodiment shown in FIG. 20 the test equipment is configured to test oranalyze an optical fiber for discontinuities, breaks, flaws, improper,dirty or malfunctioning connectors, changes in loss characteristics,unauthorized taps, or poor repairs. Collectively these may be referredto as line anomalies. While it is understood that test equipment 2004will have numerous components to enable operation, only those relevantto the invention are shown so as not to distract from the invention.Moreover, it is contemplated that the aspects and features of thisembodiment may be combined with any or all of the features of the otherembodiments and configurations discussed above. Features, method andapparatus that have already been described herein are not described indetail again.

A sequence generator 2008 connects to an optical transmission system2012 and a sequence correlator 2020. The optical transmission system2012 connects to an optical interface 2016. The optical interface 2016outputs the optical signal from the optical transmission system 2012 toan optical fiber 2028 and connects to an optical receive system 2024.The optical receive system 2024 has an output connected to the sequencecorrelator 2020. The output of the sequence correlator 2020 connects toa signal processor/analyzer 2032. Various user interfaces (not shown)may be included to facilitate use of the test equipment by an operatorand for input and output of relevant test data.

Each device is now described in greater detail. The sequence generator2008 generates or retrieves a sequence signal. The sequence generator2008 is described above in detail. The sequence signal may be generatedor retrieved from memory. The optical transmission system 2012 comprisescomponentry configured to receive and process the sequence signal fortransmission in an optical format. The optical transmission system 2012also generates and sends an optical signal over an optical fiber 2028 orfree space. In one embodiment the optical transmission system 2012includes an optical generator. The optical interface 2016 is a deviceconfigured to receive and direct optical signals to one or more outputsdepending on the port through which the signal is received and theconfiguration of the optical interface. The optical interface 2016directs the signal received from the optical transmission system 2012 tothe fiber 2028 while directing a reflection signal received over thefiber to the optical receive system 2024.

The optical receive system 2024 comprises apparatus to receive theoptical reflection signal and convert the optical signal to anelectrical signal. The sequence correlator 2020, which in thisembodiment also receives the sequence from the sequence generator,comprises apparatus to correlate the reflection signal with the sequencesignal that was transmitted over the fiber 2028. The sequence correlator2020 is described above in detail and accordingly is not describedagain. The signal processor and/or analyzer 2032 comprises any typeprocessor, ASIC, control logic, digital signal processor, or othercomputing device configured to analyze the correlated signal todetermine one or more points of correlation and perform additionalcomputing as described herein. The fiber 2028 may comprise any type offiber comprising glass or non-glass fiber or single mode or multi-modefiber or any other type of medium capable of a light signal or opticalsignal. Operation of these components is described above and below inmore detail.

FIG. 21 illustrates a block diagram of an example embodiment of theinvention configured as communication equipment. The internal apparatusof FIG. 21 is similar to that shown in FIG. 20 and hence duplicateapparatus are not described again. The apparatus of FIG. 21 isconfigured as communication equipment. It is contemplated that theoptical sequence time domain reflectometry (STDR) method and apparatusmay be configured within test equipment as shown in FIG. 20 or as partof or integral with communication equipment. Providing the STDR systemas a part of communication equipment provides the advantage of enablingSTDR operation on a line that is intended to be used for communicationor that was previously used for communication without having todisconnect the communication and connect test equipment. A furtheradvantage is that additional equipment, such as test equipment, does nothave to be purchased since the STDR system is incorporated into thecommunication system. Moreover, as the STDR system shares some of thesame componentry as certain types of communication equipment the cost ofadding STDR capability to a communication system may be less than thatwith a separate embodiment in test equipment.

In an alternative embodiment the correlation is performed in the opticaldomain, i.e. the reflection is not converted to an electrical signalprior to correlation. In one configuration of such an embodiment anoptical AND gate for each element of the N element sequence wouldprovided. In addition, an N-way splitter, N digital integrators and adigital multiplexer would be arranged to achieve optical correlation.Hence, it is contemplated that optical correlation is within the scopeof the claims that follow.

FIG. 22 illustrates a block diagram of an example embodiment of a systemconfigured to perform optical sequence time domain reflectometry(Optical STDR). Implementations or configurations other than those shownin FIG. 22 may be embodied without departing from the scope of theinvention. In the example embodiment shown in FIG. 22, an m-sequencegenerator 2204 connects to a driver 2208 and a synchronization controlunit 2279. It should be noted that it is understood by one of ordinaryskill in the art that additional component and systems would be utilizedto achieve operation, however, these additional components and systemsare omitted so as to not obscure the invention. The driver 2208transforms the sequence signal input to a signal with power level andother signal characteristics suitable to drive an optical signalgenerator 2216. Exemplary drivers 2208 include but are not limited to,an LED driver, a laser driver, external modulator driver, or integralmodulator driver, where the laser and driver may be part of a commonpart of processed single crystalline substrate, or an integrated opticalcomponent.

A resistor 2230 or other biasing device may optionally reside between anoptical generator 2216 and the driver 2208. The optical generator 2216connects to a voltage source or current source 2218. The opticalgenerator 2216 may comprise any device capable of transforming theelectrical signals from the driver 2208 to optical or light energy.Examples of suitable optical generators 2216 include but are not limitedto, an LED driver, a laser driver, external modulator driver, orintegral modulator driver, where the laser and driver may be part of acommon part of processed single crystalline substrate, or an integratedoptical component.

In this embodiment the optical interface comprises a circulator 2238.The circulator 2238 connects to or is positioned to receive the outputof the generator 2216. The circulator 2238 comprises a device configuredto selectively direct a light signal or a reflection signal to anoptical fiber 2240 or toward the receiver systems 2270 that aredescribed below. Optical fiber, a lens system, or other optical signalchanneling system 2232 may couple to the circulator 2238. The circulator2238 includes an input port 2250, an input/output port 2252 thatconnects to the fiber 2240 and an output port 2254 connects to thereceiver systems 2270. The output port 2250 receives the signal from thegenerator 2216 for transmission through the line. The input/output port2252 provides the signal to the fiber 2240 and receives the reflectionsignal from the fiber. The reflection signal exits the circulator 2238over the output port 2254. The circulator 2238 may be made to reflect aportion of the light signal from the generator 2216 to the fiber 2240and a portion to the output port 2254.

An optical fiber 2240 or other light conducting medium connects to or ispositioned to receive the output of the circulator 2238 or interface2232. The optical fiber 2240 may comprise any type fiber or mediumconfigured to carry a light or optical signal or it may comprise a lensor other system to facilitate free space optical transmission. It iscontemplated that the fiber 2240 comprise a communication channel orother fiber on which the STDR is to be performed to analyze the channelor locate a line anomaly i.e. channel anomaly.

After transmission of a sequence signal on the fiber 2240 a reflectionmay be generated when the sequence signal encounters a line anomaly. Thefiber 2240 conducts the reflection to the circulator 2238 and thecirculator diverts or directs the reflection to an optical detector2260. The optical detector 2260 receives an optical signal and convertsthe optical signal to an electrical signal. In one embodiment theoptical detector 2260 comprises a reverse bias diode or a PIN diode. Inother embodiments the optical detector 2260 may comprise any devicecapable of receiving or detecting an optical signal and converting theoptical signal to a corresponding electric signal.

The optical detector 2260 connects to a power source 2262 and anamplifier 2266 as shown. A resistor 2268 or other biasing device mayoptionally reside between the optical detector 2260 and the amplifier2266. In one embodiment the amplifier 2266 comprises a current amplifiersuch as a transimpedance amplifier. In one embodiment the amplifier 2266comprises a high-speed amplifier capable of operation at greater than 50MHz. In one embodiment the components within dashed line 2270 comprisesor operate in the manner of an automatic gain control unit to provide anoutput having a desired power or voltage level to subsequent components.

The output of the amplifier 2266 connects an analog to digital convertor2274 (A/D convertor). The A/D convertor 2274 transforms the analogreflection signal to a digital signal. Any type or resolution of A/Dconverter may be utilized. It is contemplated that the resolution mayrange from one bit to twenty-four or more bits. In one exemplaryembodiment the A/D converter operates with 14 bits of resolution. Inanother embodiment the A/D convertor may operate with 5-6 bits ofresolution. As the length of the optical fiber increases the magnitudeof the reflection decreases. Hence, more resolution, i.e. more bits ofresolution, will provide more accuracy or achieve operation whenanalyzing longer lines.

The output of the A/D convertor 2274 feeds into a sequence correlator2278 which is configured to correlate the reflection signal receivedfrom the summing junction with the original sequence signal generated bythe sequence generator 2204 to create a correlated signal. The sequencecorrelator 2278 is described above in detail and accordingly notdescribed again in great detail. The correlator 2278 outputs thecorrelated signal to a processor 2280 or other system for analysis. Itis contemplated that in one embodiment the sequence correlator is ableto obtain or generate the sequence signal as generated by the sequencegenerator 2204 and/or obtain information from the generator 2204 orother components regarding the sequence signal.

The sequence generator also provides the sequence signal to asynchronization control unit 2279. The embodiment of FIG. 22 adopts acirculator 2238. The circulator does not provide near-end echo to thereceiver systems 2270. As a result, other means to establish timing mustbe provided. In one embodiment synchronization control unit 2279monitors for the start of the sequence signal or some other referencepoint. This information is provided to the processor 2280 for purposesof timing so that a time difference between the start of the sequencesignal and the receipt of one or more points of correlation may bedetermined by the processor. As described below, the processor 2280utilizes the time information from the synchronization control unit 2279to determine a distances, i.e. location, of a line anomaly. Appropriateadjustment may occur to the timing information to account for delaycaused by the driver 2208, generator 2216, and circulator 2238, andreceiver systems 2270.

The processor 2280 may comprise any type processor, A SIC, digitalsignal processor, control logic or combination thereof capable ofperforming the tasks described herein to achieve STDR. In one embodimentthe processor 2280 analyzes the con-elated signal to perform one or moreof the following: locate one or more points of correlation, remove nearend echo if present, align the correlated signal with a template signalor original sequence signal for timing reference, and/or determine adistance or location of a line anomaly based on the time between thestart of the sequence signal, the time of receipt of the reflectionsignal at the point of correlation and the rate of propagation of thesignal through a fiber. It is contemplated that in other embodiments theprocessor may be configured to perform other operations.

Although the optical STDR system is shown in the exemplary embodiment ofa system designed to analyze or test an optical fiber, it iscontemplated that the optical STDR system may also be implemented inother configurations to achieve analysis of systems other than anoptical fiber. Hence, it is contemplated that the principles andapparatus as described and claimed herein may be utilized to performoptical STDR on other types of systems. Such systems include but are notlimited to integrated optical systems, systems or components located onan integrated circuit and systems that connect to an integrated circuit.In addition, such other systems may comprise optical interconnects thatconnect computers or other electronic devices to an optical network orwhich connect optical devices. It is anticipated that the optical STDRmay detect anomalies within devices to fraction of a millimeter or less.

FIG. 23 illustrates a block diagram of an alternative embodimentcomprising a signal alignment subsystem. As portions of FIG. 23 areidentical to portions of FIG. 22, only aspects that differ are discussedbelow. In one embodiment the circulator 2238 is configured to provide aportion of the transmitted optical sequence signal not only to the opticfiber input/output port 2252 but also to the output port 2254. Thus, insuch a configuration a portion of the original sequence signaltransmitted over the line passes through the output port 2254. While itis understood that most circulators are configured to provide completeisolation between the transmit port and the receive port, it iscontemplated that a circulator could be configured to provide onlypartial isolation between these ports.

This embodiment, as shown in FIG. 23, is in contemplation that thecirculator does not completely isolate the port 2254 from the signalpassing from port 2250 to port 2252. Hence, a portion of the sequencesignal is transmitted to the optical fiber 2240, and at the time oftransmission, a portion also passes to the optical detector 2260. As aresult, the time at which start of the sequence occurs can be determinedby transmission of the sequence signal. Consequently, thesynchronization control unit 2279 and the connection between thesequence generator and processor 2280 via the synchronization controlunit 2279, as shown in FIG. 22, may be eliminated in the embodiment ofFIG. 23. The processor 2280 will record the start of the sequencesignal, which may optionally be correlated, for timing purposes.

Through further processing by the processor 2280 or other device thelocation of the anomaly or effect that created the reflection may belocated o r determined. This is but one alternative embodiment of thereflection signal processing. Other embodiments, which do not departfrom the scope of the claims that follow, are contemplated.

FIG. 24 illustrates an alternative embodiment configured with a beamsplitter 2404. With respect to FIG. 24 and FIG. 22, similar elements areidentified with identical reference numerals. As shown, the opticalsignal generator 2216 provides an optical output to the beam splitter2404. In the embodiment shown in FIG. 24, the beam splitter 2404comprises a device configured to direct a portion of the opticalsequence signal to the fiber 2240 and a portion of the optical signal tothe optical detector 2260. An isolator (not shown) may reside betweenthe optical generator 2216 and the beam splitter 2404.

The amount or intensity of signal directed to the fiber 2240 in relationto the amount or intensity of signal directed to the detector 2260 maybe made to be any ratio or proportion as desired. In one embodiment thebeam splitter 2404 splits the signal 50% to the fiber 2240 and 50% tothe detector 2260. In another embodiment the beam splitter 2404 splitsthe signal 90% to the fiber 2240 and 10% to the detector 2260. In theembodiment shown in FIG. 24 the beam splitter 2404 is configured todirect at least a portion of the sequence signal to the detector.

In this embodiment the beam splitter 2404 provides a portion of thegenerated sequence signal that is being transmitted over the fiber 2240to the detector as a near-end echo signal. Hence, the receivercomponentry has access to the original sequence signal in the form ofthe near-end echo. Processing on the near end echo, which may be usedfor timing reference and alignment, may occur as describe above.

Upon receipt of a return reflection signal the beam splitter 2404directs at least a portion of the reflection signal to the detector2260. Other embodiments may direct differing percentages of thereflection signal to the detector 2260. The reflection signal may be aweak signal and hence it may be desired to direct as much of thereflection as possible to the detector 2260. The beam splitter 2404possesses features or characteristics that may make it desirable foruse. One such desirable characteristic is that it may cost less than acirculator. Another desirable feature is that it may be made to providea portion of the sequence signal being transmitted over the line 2240 tothe detector 2260 for timing purposes. Other advantages of the beamsplitter 2404 over the circulator include ease of integration with aMEMS structure due to the simplicity of manufacturing processes and thesimilarity of materials in present usage.

FIG. 25 illustrates a block diagram of an optical interface 2504. Thegeneralized optical interface 2504 is a functional representation of adevice capable of directing the generated sequence signal from a signalgenerator to an optical fiber while directing a reflection received fromthe optical fiber to the receiver components, such as to an opticaldetector. From a functional viewpoint, the optical interface 2504comprises a first port 2508, a second port 2512 and a third port 2516.It is contemplated that for purposes of discussion and understanding thefirst port receives a signal, such as for example an optical signal,from a transmit module 2520. The second port 2512 may connect or coupleto a channel, such as an optical fiber 2524 while the third port 2516may connect or couple to a receive module 2528, such as an opticaldetector. It may be desired to fully or partially isolate the signalgenerator, such as an optical generator, from the receive module, suchas an optical detector so that the power level of the transmitted signaldoes not harm the sensitive receiver module. Hence when a signal isprovided through the first port 2508 to the second sport 2512 it may bedesired to control the amount of signal that is provided to the thirdport 2516. Similarly, if a reflection signal is received over a channelor optical fiber by the second port 2512, then it may be desired todirect the reflection signal to the third port 2516 while directing thereflection signal away from the first port 2508.

As shown the first port 2508 may receive a signal represented by I_(t),where I_(t) represents the intensity of the transmitted signal. At theoptical interface the intensity of the transmitted signal passing out ofthe second port 2512 is represented as α₁I_(t) and the intensity of thesignal passing out of the third port 2516 is represented as α₂I_(t). Thevalues α₁ and α₂ represent the percentage of the signal that the opticalinterface 2504 directs to each of the other ports when presented with asignal from the transmit module 2520.

A reflection signal received at the second port 2512 is represented asI_(r). After passing through the optical interface 2504, the reflectionsignal with intensity β₂I_(r) passes out of the third interface 2516while the reflection signal having intensity β₁I_(r) passes out of thefirst interface 2508. The coefficients β₁ and β₂ represent a percentageor an amount of the original intensity of the signal received by theoptical interface 2504 that is passing out of a partial port opticalinterface. In one embodiment, where losses of the optical interface 2504are ignored, the following equations define a relationship between thecoefficients.

 α₁+α₂=1β₁+β₂=1.

In one embodiment where the optical interface comprises a circulator thevalue α₂ is very small. In the embodiment of a 50—50 beam splitter thevalues for all α and β are 0.5. In one embodiment if the value of β₁ isother than a very small value then an isolator may be required betweenthe transmit module 2520 and the optical interface 2504. If the value ofα₂ is other than a very small value then an isolator may be requiredbetween the receive module and the optical interface 2504. It iscontemplated that the value of α and β may be made to be any valuedesired. Amplification may be integrated into the optical interface2504.

FIG. 26 illustrates a flow diagram of an example method of operation ofan optical STDR system. The steps of FIG. 26 may be combined in anycombination with the steps of other methods described herein. Certainsteps may not be executed. This is but one example embodiment. It iscontemplated that other methods of operation are possible and within thescope of the invention as define by the claims. This exemplary method ofoperation may occur in either communication equipment or test equipmentconfigured to perform STDR. At a step 2602, the sequence time domainreflectometry system (hereinafter system) generates a sequence signal orretrieves a sequence signal from a memory. The sequence signal maycomprise an M-sequence or any other type of sequence. In one embodiment,the sequence comprises a sequence with good autocorrelation properties.Various different types or classes of sequence signals may be generatedor retrieved from a memory. It is contemplated that a user or the STDRsystem may generate and use different types of sequence signals. At astep 2604, the operation performs signal mapping to assign the sequencesignal to one of several different values. As is understood in the art,the signal mapping occurs when the sequence signal is generated by asequence generator that generates logic levels. In the event that thesequence is stored and retrieved from memory at the time of use, mappingmay not occur since the proper sequence, in a pre-mapped form, would bestored.

At a step 2606, the system filters the signal to remove unwantedcomponents. At a step 2610, the system provides the sequence signal toan optical signal source driver. In one embodiment this comprisesproviding the sequence signal to a driver that generates a signal thatis provided to a laser or LED to create an optical signal. At a step2612, the system generates an optical signal based on the output fromthe driver. In one embodiment the signal is generated by a laser. Inanother embodiment a LED generates the signal. It is contemplated thatthe signal is provided to the optical fiber or to an interface device,such as a circulator or a beam splitter. The signal is then transmittedover the optical fiber or free space optics. It is understood thattransmission of a signal over a channel will generate reflections atpoints of line anomalies.

Thereafter, at a step 2614, the system monitors for and receives anyreflection signals generated by the transmission of step 2612. Thereflection signal may be defined as the signal(s) received during aperiod of time after the transmission of the original sequence signalover the channel. Thus, the reflection signal may actually compriseseveral periods of no signal and one or more individual echoes createdby the sequence signal encountering line anomalies as it travels downthe optical channel. The reflection signals created by the sequencesignal may be received at different times after transmission.

Upon receipt the reflection signal is converted from an optical formatto an electrical format. This occurs at a step 2616. The reflectionsignal, once in electrical format, may be stored for processing at alater time or processing may continue at step 2620 by filtering thereflection signal to remove signals at unwanted frequencies. At step2622, the system correlates the reflection signal with the originalsequence signal. The correlation may reveal one or more points ofcorrelation within the reflection signal which appear as peaks or pointsof increased magnitude.

Stated another way, the channel is monitored after the transmission ofthe sequence signal for a period of time sufficient for any reflectionsgenerated by the transmission to be recorded by the monitoring. Thereflection signals received during this period of time are convertedfrom the optical to the electrical domain and stored or processed.Correlation occurs at step 2622 using the original sequence signal andany signals recorded during the monitoring period of step 2614. Peaks inthe correlated signal occur at the points of time in the monitoring whena reflection from a line anomaly was received.

At a step 2624, the system synchronizes the correlated signal with thestart of the sequence. This allows for an identification of a time, inrelation to the start of the sequence signal transmission or otherreference point, at which points or correlation, i.e. reflections,occur. Hence, the time between when the sequence signal was transmittedand when the reflection was received, i.e. a point of correlation, canbe determined. The peak of the near-end echo may serve as the referencetime point for this calculation.

At a step 2626, the system removes unwanted artifacts or disruptivereflections. One example of a disruptive artifact is near-end echo.Removal of unwanted artifacts may occur by subtracting a templatesignal. Thereafter, at a step 2630, the system analyzes the correlatedreflection signal to determine the location of line anomalies. This mayoccur by first processing the reflection signal to determine the timedifference between the start of the sequence signal transmission and thepeak of a point of correlation of the correlated reflection signal.

In one embodiment utilizing Golay or m-sequences, the relative locationof the source equipment or transmitter of the Golay or m-sequences maybe obtained from the NEAR-end-echo. Location of the FAULT is usually maybe identified from the FAR-end-echo. Identification of the far-end-echomay require elimination of the near-end-echo after the relative locationof the source equipment has been obtained from the processed reflectionsignal. Thus, it is possible to eliminate the near-end-echo sourceequipment location step.

Next, the time difference may be multiplied by the rate of propagationof the signal through the channel. The rate of propagation of a lightsignal depends on the type of optical fiber or free space optics thatare used for the channel. An optical signal propagates at about 50% ofthe speed of light while in multimode fiber and at about 80% the speedof light when in single mode fiber. These calculations provide thecombined distance to and from the line anomaly using the transmitter orline interface as a reference point. Since the optical sequence signalmust travel to the anomaly and return in the form of an echo, thedistance value may be divided by two to arrive at a true distance to theanomaly.

It is contemplated that certain communication systems, electronicapparatus, hardware systems, software systems, memory devices,registers, or any other communication oriented device may be unable toaccommodate one of the above described sequences. In addition, othersystems may be unable to accommodate an odd length sequence, such as anm-sequence, or only accommodate sequences of certain length. By way ofexample, all m-sequences are of length 2^(n)−1 which yields an oddlength sequence. If a system is only able to process or store an evenlength sequence, such as for example, a sequence of length 512 or 1024points, then an m-sequence or other odd length sequence may not be used.As a drawback, the correlation processing described herein may not beeffective.

As a result, it may be desirable to establish operation with a differentset class of signals. Based on the disclosure herein, the method andapparatus for line probing may also be performed with complementarysequence pairs in addition to the above described signals or sequences.Examples of complementary sequence pairs comprise Golay codes,multi-level sequences or any set of sequences, or any other type of twoor more sequences that can be processed, either before or aftercorrelation, to yield an impulse response. In one embodiment,complementary sequences are any pair of sequences for which the sum oftheir correlations yields an impulse function. It is contemplated thatother sequences may be utilized and combined or processed in any mannerof ways to yield a correlated result from which an impulse response maybe obtained. Types of processing other than that shown below may occurto obtain a combination that has the impulse response. As discussedbelow, other types of processing may occur.

For example and in reference to FIG. 4, the sequence correlator 446 orthe analysis module 450 may comprise or include memory or registersconfigured to store the two or more correlated signals. Similarly theanalysis module 450 or the sequence correlator 446 may be configured tocombine or process the correlated signals as described herein.

Hence, it is contemplated that more than two sequences may betransmitted and processed in any manner to generate a correlated outputthat can yield an impulse response. Likewise, it is contemplated thatinverted sequence pairs may be utilized. Examples of complementarysequences are discussed in more detail in Chapter 13 of the bookSequence Design for Communication Applications by P. Fan and M. Darnell,offered by John Wiley Publishing, copyright 1996, which is incorporatedin its entirety herein.

That the signals described herein may be utilized may be shownmathematically starting with the known principles for m-sequences.Assumingm(n, N)*m(−n)*=δ(n, N)(Where m(^(Q)n)* is the time reversed complex conjugate of m(n), and *represents the convolution operator. This is known as correlation.Another note, the correlation doesmay not yield the Kronecker deltafunction, which is described bellow, but may yield a scaled version ofit depending on the period of the m-sequence. Namely, Nδ(n, N)!. In thecase of the Golay codes and, after correlation the combination may ityields 2Nδ(n, N)!. In addition, the Kronecker delta function is thediscrete counter part of the continuous time Dirac delta function. Inthis equation m(n) is the m-sequence, which is transmittedintermittently or continuously, and m(n,N) is the periodic repetition ofan m-sequence. Thus, the m-sequences have the desirable function thatthe m-sequence correlated with the periodic repetition of an m-sequenceyields the function δ(n,N) which is a delta function. The delta functionis an all zero response with a pulse or unit output at the points ofalignment during correlation. This generally occurs every N points,where N is the period. Hence this relationship is of importance, sincewhen this relationship between convolved signals is satisfied, theprinciples of sequence time domain reflectometry as described above maybe realized.

In support of this principle, the received signal may be represented asr(n)=t(n)*h(n)wherein r(n) is the received signal, t(n) is the transmitted signal, andh(n) represents the impulse response of the channel. Thus, convolvingthe transmitted signal with the impulse response of the channel yieldsthe received or reflected signal.

Thereafter, the reflected signal may be correlated with the originalm-sequencec(n)=t(n)*h(−n)*wherein m(n) represents an m-sequence, to yield the correlation outputc(n).

Through substitution and based on the linearity of these equations,c(n)=m(n, N)*h(n)*m(−n)*and can be re-written asc(n)=h(n)*[m(n, N)*m(−n)]which in turn reduces toc(n)=h(n)*S(n,N)This is a periodic impulse train and is thus the periodic extension ofthe echo channel to the impulse train. Hence, the output of thecorrelator is the impulse response of the channel that is periodicallyrepeated, which is defined by the following equation:c(n)=h(n, N)From this, it can be seen that m-sequences satisfy the requirement. Butthere are not any even length sequences that have this property, andhence, for the situation of a system able to only accommodate evenlength sequences, the m-sequence signal time domain reflectometry maynot be an option. This would comprise a drawback, but turning tocomplementary sequence pairs, it can be shown mathematically that ifcomplementary sequences g₁(n) and g₂(n) are selected and defined as evensequences, i.e., sequences with an even number of points, then thesedrawbacks are overcome.

Stated mathematically, it can be shown thatg ₁(n,N)*g ₁(−n)*≠δ(n,N)for any A, which is simply the correlation of two even sequences. Thisdoes not equal the delta function and hence lacks an important propertynecessary for the process described above. However, by combining twoeven sequences in a desired manner, the correlation of the evensequences may result in the delta function. Hence, g ₁(n, N)*g _(n)(−n)+g ₂(n, N)*g ₂(−n)*=δ(n, N)Thus, by combining such sequences, the beneficial correlation propertiesdescribed above may be realized. In one embodiment, the combinationcomprises an addition of the two complementary code's correlation.

Therefore, in one embodiment, the periodic extension of g_(i) may betransmitted and correlated with g_(i) in the receiver. Thus, the firstsequence g_(i) of a complementary sequence pair is transmitted, receivedand correlated with itself. This is shown asc ₁(n)=g ₁(n, N)*h(n)*g ₁(−n)*where g₁ is a first complementary code and h(n) is the impulse responseof the channel. Thus, the first complementary code of a complementarypair is transmitted through the channel, and the reflection iscorrelated with the original sequence signal. The correlation operationon a signal is described above in detail and hence, is not describedagain. The channel may comprise any channel as described above.

Based on principles of linearity, the equation may be re-written asc ₁(n)=h(n)*[g ₁(n,N)*g ₁(−n)]Likewise, this operation may be repeated with the periodic extension g₂and the received reflection is correlated with the originallytransmitted sequence g₂ in the receiver. Thus, the second sequence g₂ ofthe complementary sequence pair is transmitted, the reflection receivedand correlated with itself. This is shown asc ₂(n)=h(n)*[g₂(n,N)*g ₂(−n)]where c₂(n) is the second correlated signal created by transmission ofthe second sequence g₂ through the channel and the received reflectionis correlated with the original second sequence g₂. The process ofgenerating the correlated signal c₂(n) may be considered a secondoperation to createc ₂(n)=h(n)*[g ₂(n, N)*g ₂(−n)*]

Through mathematical manipulation of these linear systems, it can beshown thatc 1(n)+c 2(n)=h(n)*[g ₁(−n)*g ₁(−n)*]+h(n)*[g ₂(n, N)*g ₂(−n)*]which reduces due to linearity toc1(n)+c2(n)=h(n)*[g ₁(−n)*g ₁(−n)*+g ₂(n, N)*g ₂(−n)*]This equation can be re-written asc 1(n)+c 2(n)=h(n)*δ(n,N)=h(n, N)by substituting in the complementary sequence correlation propertydescribed above. This is the base fundamental relationship discussedabove, and thus the resulting summation of the two convolved signal willyield the desired channel information. Thus, this method may be used todetermine channel characteristics or any other operation as describedherein, even when the system executing such processing may not utilizem-sequences or an odd length sequence.

It is contemplated that the either or both of the complementary codes,i.e., sequences, may be transmitted continuously or periodically. In oneembodiment, the complementary codes are transmitted continuously tothereby gain the benefit of easier synchronization and alignment of thesignals. As a result, the complex timing issues of when to startmonitoring and when to stop monitoring are eliminated, as are complexand large buffers. In such an embodiment, a clean period of thereflection signal, after correlation, may be captured at any pointduring the continuing transmission. In one embodiment, three sequencesare sent and the middle used for correlation. This provides theadvantage of a less expensive and less complex timing control system.

FIGS. 27A and 27B illustrate an operation flow diagram of an examplemethod of operation. This is but one example method of operation, and itis contemplated that other methods of operation may be enabled withoutdeparting from the scope of the invention. At a step 2704, the operationinitiates a channel analysis operation to characterize the channel anddetermine if a line anomaly is present or where a line anomaly islocated in relation to the point of testing. Thereafter, at a step 2708,the operation specifies a sequence or signal for use in the analysis. Inthis example embodiment, the signal comprises two or more complementarysignal pairs. At a step 2712, the system obtains the first signal of thetwo or more complementary signals. As discussed above, the signals couldbe generated or retrieved from memory.

Next, at a step 2716, the system transmits the first signal through thechannel, and at a step 2720, the system monitors for and receives, at astep 2724, a first reflection signal generated by the first signalencountering a line anomaly. The signal may be transmitted continuously,and as a result, the reflection signal may be continuously received.Thereafter, at a step 2728, the operation correlates the firstreflection signal, with the transmitted first signal, to create a firstcorrelated signal. The correlation operation is discussed above in moredetail. The correlated signal may be stored or otherwise saved for useas discussed below.

Turning now to FIG. 27B, at a step 2736, the system obtains the secondsignal of the two or more complementary signals and at a step 2740transmits the second signal through the channel. The signal may betransmitted continuously, and as a result, the reflection signal may becontinuously received. Thereafter, at steps 2744 and 2748, the systemmonitors for a reflection and receives a second reflection signal. As aresult of the continuous transmission, the reflection may also bereceived continuously. At a step 2752, the system correlates the secondreflection signal, with the transmitted second signal, to generate asecond correlated signal. This type of operation may be repeated at step2756 to generate numerous correlated signals as may be desired toobtain, after processing, at step 2760, a final result having propertiesof an impulse response. In the particular case of Golay codes, aliasembodiment, it is preferred that the two complementary sequence'scorrelation need to be aligned in time before they are combined throughaddition.

At a step 2760 of this embodiment, the operation combines the firstcorrelated signal with the second correlated signal and any additionalcorrelated signals generated at step 2756. In one embodiment, thecombination comprises adding the two or more correlated signals toobtain a result that approximates a delta function as described above.Thus, through the combination of the correlated reflections of two ormore transmitted signals, a combined or processed signal may begenerated that has behavior or characteristics of a delta function.Thus, the combined correlated signal behaves or is similar to theresponse of a correlation of a reflected or correlated m-sequence.Hence, systems or software that are unable to transmit m-sequences, thepreviously described types of signals, or odd length sequences may nowbe accommodated using a combination of signals. When transmitted, thesignals generate a reflection that can be correlated and processed, andoutput and have an impulse response of the channel plus reflections andnoise.

At a step 2764, the system may optionally initiate artifact reduction onthe resulting signal. It should be noted that the combination of thecomplementary sequences correlation is the correlation artifactreduction in the Golay complementary codes case. At a step 2768, anear-end echo reduction routine may optionally be initiated. At step2772, the operation initiates processing on the combined correlatedsignal to determine the location of the line anomalies or tocharacterize the line. The various aspects of step 2772 are discussedabove, and hence are not described again.

While various embodiments of the invention have been described, it willbe apparent to those of ordinary skill in the art that many moreembodiments and implementations are possible that are within the scopeof this invention.

1. A method for performing time domain reflectometry on a communicationchannel comprising: obtaining a first complementary signal; transmittingthe first complementary signal over a communication channel; receiving afirst reflection signal from the communication channel in response tothe transmitting of the first complementary signal; correlating thefirst reflection signal with the first complementary signals to generatea first correlated signal; obtaining a second complementary signal;transmitting the second complementary signal over a communicationchannel; receiving a second reflection signal from the communicationchannel in response to the transmitting of the second complementarysignal; correlating the second reflection signal with the secondcomplementary signal to generate a second correlated signal; processingthe first correlated signal and the second correlated signal to generatea combined correlated signal; determining a point of alignment betweenthe transmitted signals and the combined correlated signal; measuring atime interval between the point of alignment and a subsequent peak inthe combined correlated signal; and multiplying the time interval by therate of propagation of the sequence signal through the communicationchannel to obtain distance information regarding a line anomaly.
 2. Themethod of claim 1, wherein a peak in the combined correlated signal iscaused by a bridge tap.
 3. The method of claim 1, wherein thecommunication channel comprises a twisted pair conductor.
 4. The methodof claim 1, wherein transmitting the first and second complementarysignals occurs at a power level that does not introduce crosstalk intoother communication channels.
 5. The method of claim 1, furthercomprising performing a circular rotation of the complementary signalsto create two or more rotated complementary signals; transmitting thetwo or more rotated complementary signals over the communicationchannel; receiving two or more rotated reflection signals; correlatingthe two or more rotated reflection signals with the two or more rotatedsequence signals to create two or more rotated correlated signals;aligning the two or more rotated correlated signals with the two or morecorrelated signals; and adding the two or more rotated correlatedsignals to the two or more correlated signals to reduce or removecorrelation artifacts on the two or more correlated signals.
 6. Themethod of claim 1, further comprising: retrieving a template signal;aligning the template signal and the combined correlated signal; andsubtracting the template signal from the combined correlated signal toremove near-end echo from the combined correlated signal.